Fort Vol. 2 1976 Planning versus fortification: Sangallo's project for the defence of Simon Pepper

Since 1527, when Rome had been captured and sacked by the mutinous soldiers of Charles V, it had been clear that the defences of the Papal capital were hopelessly outdated. The walls of the (the Vatican precinct) were constructed during the pontificate of Leo IV (847-855): those of and the left bank, enclosing by far the largest part of the city, dated from the reign of the Emperor Aurelian (AD270-75) [1]. Impressive both for their length and antiquity, these walls were poorly maintained and fundamentally unsuitable for defence against gunpowder artillery. In 1534 the Romans were once again forcefully reminded of their vulnerability when a large Turkish fleet moored off the estuary. Fortunately the hostile intentions of the Turks were directed elsewhere: after taking on fresh water they sailed north to raid the Tuscan coastline. But in the immediate aftermath of the Turkish scare the newly elected Paul III committed himself to an ambitious scheme of re-fortification. Antonio da Sangallo the Younger, advised by many of the leading architects and soldiers employed by the Pope, was commissioned to submit design proposals [2]. Father Alberto Guglielmotti, the nineteenth-century historian of the Papal armed forces, tells us that Sangallo and his consultants decided to replace the Aurelian wall with a new line of works defending the developed areas on both banks of the river. The 18000 metre Aurelian circumfer- ence was to be reduced by half, a decision which is not difficult to understand when one glances at a contemporary map of the city. Mediaeval Rome was crowded into the district — the ancient Field of Mars in the bend of the river — with village scale outposts in the Borgo, Trastevere and around the . The fifteenth and sixteenth century additions extended north towards the and east to the hi I Is—collectively known as the - where many Popes, among them Sixtus V, attempted to promote urban development. However, mediaeval and renaissance Rome together occupied only a small proportion of the fortified area. Other European cities contained sizeable areas of open land within their walls; but none rivalled Rome's 'urban countryside' dotted with ruins, farms, vineyards, villas and the strongholds of the nobility. Obviously this underdevelopment supported the case for drastic reduction of the circumference. Sangallo's team proposed a new circuit fortified by 18 immensely powerful double-flanked bastions, spaced at intervals of about 600 metres, supported by intermediate gun platforms (a type of blunt headed bastion). Guglielmotti's description is based in part upon accounts published by contemporary theorists, the best being that of Francesco de Marchi [3], and, in part, upon a Sangallo sketch showing an idealised version of the arrangement. Very little of this scheme was actually built. After 1542 it was decided to concentrate resources on the fortifications of the Borgo — the Papal enclave on the right bank which included 33 SIMON PEPPER

Sangallo Bastion: Junction of curtain and flank.

St Peter's Basilica, the Vatican Palace and the Castel San Angelo. By 1542 only one bastion had been finished near the , at the southern tip of the circuit, another partly built on the north peak of the Aventine, and a third at a very early stage of construction on the south peak of the Aventine [4], However, the abandoned scheme made a profound impression upon the military architects of the period. As Horst De la Croix has recently pointed out, the prestige of the project attracted the interest of the entire military architectural profession, so that treatises published many years later contained references to ideas that first emerged from the 'conferences' convened by Paul III [5], Even in its reduced form it was probably the largest fortification project SANGALLO'S DEFENCE OF ROME

Mini's Plan of Rome of 1551. (North is to the left): Leonardo Bufalini published one of the first printed maps of Rome in 1551, based on a measured survey hrtaken in 1534-35 in support ofSangallo 'sfortifiations. For a discussion of the plan and its origins see: Francesco Ehrle, La Pianta di Roma al tempo di Giulio Home 1911. SIMON PEPPER 36

Sangallo diagram illustrating the double bastion system: This cropped Sangallo sketch ( Uffizi, 942) seems to illustrate the early proposals as described by Francesco De Marchi. It is probably a theoretical diagram: certainly there is nothing to connect it with any Roman topographical feature. There are also significant differences between it and the competed Ardeatine Bastion. The acute-angled bastion tips of the diagram are expanded in the built scheme, giving more room for the face batteries (that is, the guns shooting away from the walls). The flank batteries, shooting along the walls, were also enlarged in the final scheme: the central 'platform' and demi bastions were eliminated.

The Pozzi, well-shafts, Sangallo's drawing (Uffizi, 1505A) shows the lowest countermine level with the masonry tinted. Plan of countermine gallery and rampart sections. (Uffizi, 1362A). This could be called a working drawing, with dimensions, scale and set-outs to the existing Aurelian Wall.

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Three dimensional study of Ardeatine bastion: (Uffizi, U936A). The lower drawing is easily recognised as the completed bastion, although nothing remains of the free-standing cavalier, and the parapet details have been changed. The caption 'Per lafronte di S Antonino', refers to the sector before the Baths ofAntoninus Caracalla. The upper drawing is always catalogued as an aternative proposal for the same sector. However, it bears a strong resemblance to the hornwork on the Pincio which, viewed from the north, has a large round tower in mid-curtain, probably built over the Capozocha? SIMON PEPPER

of a century which had yet to see the new city of Valetta or the great fortress of Palmanova. Moreover, the design of the completed bastion was most advanced. Scamozzi, Palladio's follower, writing in 1615 tells of 'that most famous bastion at Rome, with double flanks, both high and low level gun platforms, countermines, casemates and well-shafts, with a wall of such height and breadth that it cost a great deal of treasure' [6], Recently the vegetation which disfigured this bastion has been cleared away to reveal a most impressive if sombre, example of the forms evolved during the sixteenth century to resist the onslaught of gunpowder artillery. But the sophistication of Sangallo's design can best be appreciated from modern drawings which dissect the bland, brick-clad shapes to show the internal features mentioned by Scamozzi. Although the scheme itself was abandoned, some of Sangallo's drawings have survived. They are preserved in the Uffizi Gallery but, despite publication in facsimile by Enrico Rocchi in 1902, have attracted very little attention [7]. No doubt this is because they are strictly utilitarian documents; ranging from preliminary topographical studies through sketch layouts, to outline working drawings. As works of art they do not bear comparison with the bravura sketches of

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Roman fortifications. Key: A, Aurelian wall (3 rd century), B, Leonine wall (9th century), C, Sangallo's proposed circuit (1534-42), heavy solid line shows the route indicated on the Uffizi drawings, D, Paul Ill's Borgo fortifications constructed after 1542, probably part of 1534-35 scheme, E, Urban VIII's fortifications (1623-44). SANGALLO'S DEFENCE OF ROME

Kline bastion. Key: A, Cavalier: an upper level gun platform. The cavalier must have had an access ramp, but no trace of it remains. B, piazza: an nm platform housing large pieces. C, vaulted bomb shelters. D, casemate: an enclosed gun chamber acommodating relatively small pieces of artillery, rraMvbreech loaders. E, magazine. F, ventilation flues. G, countermine gallery.. H, pozzi (well-shafts). J, countermine shaft. K, sally port. L, special ingle embrasure for fling at ground close to walls. pllo's 'double bastion' was highly esteemed for two feaures: the weight offlanking fire that could be brought to bear across a breach, and the istication of its underground countermines. In an emergency guns could be accommodated in each flank as follows: 2 small pieces at piazza level: 1 medium pieces mounted en barbette at both cavaliers. Thus, in a symmetrical system, four flanks and two cavaliers could generate a cross fire of Vis (see fig. 15). Mines offered an alternative means of attack against strongly armed works. Shafts would be driven beneath them and then, either ipsed by firing the pit-props, or exploded with even more dramatic effect. Countermines were devised to detect and frustrate such projects. Listening would be established at the end of each underground shaft: the sentry would be locked in with an ear-trumpet, candle, a drum with pebbles on it, and rlislening devices. Once a fix' had been obtained, the shaft could be extended towards the enemy mine which could be broken into by a raiding party l/stroyed by 'depth charges' laid ahead of it sandbagged, and fired by long fuses. Underground fighting varied very little from ancient times to the thes of the Great War. SIMON PEPPER

Double bastion compared to other bastion plans: This diagram is a potted history of 16th century military architecture. A: The medieval tower accommodating small numbers of guns, with a 'blind spot' in front B: Early 16th century bastion, triangular in plan to eliminate the blind spot, with increased space for artillery C: Later bastions with blunt profiles giving more room for face batteries. Flankers located in so-caled traditore (traitor) batteries, protected from frontal fire by the shoulders of the bastion. Increased flanking fire could be achieved by using tiers of traditori. Sangallo's double bastion system falls chronologically between B and C. Enormous firepower is achieved by using two flanks per bastion, both built to give multiple tiers. However, its complicated form could only be constructed in masonry, as opposed to earthwork with a brick skin, and its artillery positions were both more confined and less flexible than those of the 'basic' bastions C.

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Michelangelo [8], or the fine woodcuts and engravings which illustrate many of the published treatises. Indeed, they are scruffy documents and very difficult to read, many of them having two or three alternative layouts drawn on top of each other with progressively heavier lines. They do, however, bring us face to face with one of the more interesting problems surrounding the project: namely, that the scheme indicated by Sangallo's drawings bears no resemblance to that described by Guglielmotti. Guglielmotti reported the decision to halve the Aurelian circumference but, while the drawings do not give a complete picture of the proposed circuit, it is clear from those parts which can be established that a much more modest reduction was finally agreed. The line of Sangallo's walls amount to a refortification of the Aurelian line. Moreover, none of the gun platforms and only one of the promised 18 double-flanked bastions can be identified. Here again are the components of the problem: 1. Winter 1534-35: a conference of experts agrees to halve the Aurelian circumference, apparently a most sensible decision on both military and economic grounds. 2. Sometime shortly afterwards this proposal is shelved. The evidence: Sangallo's drawings and the fact that some of the building work started far away from any possible short route (ie at the extreme southern point on the Aurelian wall). 3. The problem: why should the short, economic route be abandoned in favour of a plan to re-fortify most of a very long wall (18 000 m) enclosing a medium sized city (population between 30 and 40 000 in the mid-1530s). SANGALLO'S DEFENCE OF ROME

Sangallo's final proposalfor the A ventine: This drawing is based upon the most developed ofSangallo's A ventine proposals. On it the architect has shown both the location and orientation of the embrasures and, when lines of fire are added, it can be seen that he was using two systems of fortification. On the northern Aventine Sangallo projected conventional bastions from the walls, giving flanks from which guns could sweep all external faces. Between the peaks and to the south of the southern Aventine, however, he created re-entrants so that the main protection for any face came, not from the relatively small fanks, but from all along the adjacent face. This device was repeated for the short cut south of the Lateran.

The historian's first response to this kind of difficulty is to check that the drawings, descriptions and abandoned buildings all relate to the same scheme. Of this there can be no doubt. Details and dimensions of the Ardeatine bastion and the incomplete works on the Aventine correspond precisely to information in the drawings; and although archival data is patchy, there is sufficient evidence in the form of payments for services, materials and building work to connect the surviving fortifications with Paul Ill's project [9]. Further, there can be little doubt that the authors cited by Guglielmotti described the same initial proposal to reduce the circumference and, with it, the cost of fortifications, troops and artillery needed to defend Rome. Recalling Scamozzi's remark about the 'treasure' consumed by the Ardeatine bastion, we are bound to ask why this apparently logical plan should have been abandoned. Were there, perhaps, insuperable technical difficulties which made the short route unviable? To answer this question we must put ourselves into the position of a fortification designer and consider the topography of the city. Inhabitated Rome was built between the seven hills, not on them. Yet to defend the inhabitated area it was essential to hold the high ground to prevent new fortifications from being overlooked. To be sure, many of the Roman monti were not very high, but in siege warfare, as in modern trench warfare, any overlooking was important. Thus no retreat from the Aurelian line could be allowed to abandon the commanding positions of the Aventine, to the south-west, and SIMON PEPPER

joining the Castel SAngelo, the , the church ofSTrinita dei Monti, the Templum Solis and the , going from west to east. The first scheme simply follows this line with a bastionedfront. Both the second and third shemes push salients northwards along the Pincio to a feature called the 'Capozocha' which is believed to have been the circular plinth to an ancient sacred building. Sangallo's notes explain the need for the salient. Just to the north of the Capozocha he says: 'This must be held to forbid the place to the enemy who would attack below'. Other references to the 'ground below' make it clear that the Pincio was to be defended to prevent an enemy siting guns on it in support of attacks against the low positions near the Mausoleum. The final scheme shows the salient front broken to form two flanks, each with casemated guns sweeping much shorter faces at close range. This is the so-called Tenaglia or Hornwork. Monte Pincio, to the north. Elsewhere the designer could exercise some choice. From the Pincio to the Aventine the Aurelian wall ran continuously on high ground except at two points on either side of the Lateran Palace. But by clinging to the high ground only modest short cuts could be achieved. The best of these was to fortify the base line of the triangle formed by S Croce in Gerusalemme, on the eastern tip of the ward, the Pincio, and the Castro Praetorio — the old Imperial Roman fort which formed a rectangular bump on the north-eastern extremity of the wall. About 1000 metres would be saved. To retreat inside this line would demand fortifications climbing up and down across the pattern of ridges and valleys that reads on Bufalini's map as a series of fingers spreading out from the . A zig-zag layout would probably be used with the parts on the ridges pushed forward towards an enemy so that any attack against the lower parts of the wall would be taken in flank from both sides This crossfire, of course, was the guiding principle of the Renaissance system of fortifications planned so that guns in the flanks of each bastion swept the forward faces of adjacent bastions, as well as the ground between them. Bastions were triangular in plan, not, as is sometimes supposed, because the sharp point deflected shot but because rounded or squared tips would always give an enemy the use of a small blind spot of unbeaten ground in which to conceal himself from the flankers [10]. However, the difficulty at Rome was that the ridges were too far apart to be used as sites for flanking batteries using sixteenth-century cannon. Most authorities agreed on bastion-to-bastion spacing of 200 to 300 Papal coal of arms on bastion salient SIMON PEPPER

metres: the tops of the ridges are at least twice that distance apart. Sangallo faced the problem of a series of salients defending their own fronts without the benefit of supporting fire from other 44 parts of the line. His drawings for the works on the Pincio and the Aventine show that he was capable of a brilliant design solution to this problem, developing a work he called a tenaglia but which was known to later generations of military architects as the hornwork. Sangallo was thinking in terms which would have permitted a short-cut from the Aventine to a point on the Esquiline near S Maria Maggiore and from there across the Viminale and the Quirinale ridges to the Pincio. The short route was possible, but is marked on my plans in a broken line as befits its hypothetical status. Yet whatever the possibilities and initial intentions may have been, the bastion constructed by the Porta Ardeatina obviously formed part of a scheme to refortify all but small sections of the Aurelian circuit. And if military factors alone are to be considered, it is almost impossible to justify a change of plan which would seem to destroy the reasonable proposals formulated during the winter 1534-36. A town planner, however, would probably not be surprised at the change of plan. Although the development of the Quirinale ridge, which included Michelangelo's scheme, did not start until the early 1560s, it was clear by the mid-1530s that the long-term future of the planned city lay on the empty high ground [11], The monti were considered the healthiest parts of the city. Since the 1480s tax concessions and other privileges had been offered to those willing to build in the monti: only a proper water supply was missing until Pius-IV (1559-66) repaired the Aqua Vergine viaduct. From a planner's point of view these factors would support a decision to keep the monti within the walls. The short fortification route, on the other hand, would exclude not only

Double flanks of Sangallo bastion. SANGALLO'S DEFENCE OF ROME the potential building land on the monti but the church of S Croce and, more important, the Lateran Basilica and Papal palace. Sangallo's proposal for the northern front was also likely to be controversial. His drawings 45 for this sector show that the old wall between the Pincio and the river was to be pulled back southwards to a new line joining S Trinita dei Monti to the Mausoleum of Augustus. In 1534 the land between the proposed new line and the Piazza del Popolo was not yet fully developed; although Bufalini's plan of 1551 shows it to be one of the best laid-out districts and other plans indicate intensive building activity during the middle years of the century. Before water was piped to the monti this level ground south of the Piazza del Popolo was probably the most valuable building land in Pauline Rome. The piazza itself was the site of an important market and the starting point for the horse races which gave the present its name. The main pilgrim and trade route entered Rome through the and it was to improve communications between that gate and the Borgo that Leo X (1513-21) constructed the Via Leonina, today the Via Ripetta. Everyone with property in this area could be expected to oppose Sangallo's scheme. No such considerations applied to the deserted district behind the where work began on the double-flanked bastion. Nor is there anything to suggest that the southern

Palmanova (1593). SIMON PEPPER

Curtain and Flank of Sangallo Bastion Salient of Sangallo bastion. sector of the wall was more dilapidated than others. Indeed, the soldiers who sacked Rome in 1527 forced their way into the city at a relatively modern point on the Borgo wall [12]. All things considered, it seems possible - even probable — that the first stage was implemented at the least sensitive site. The decision to begin work at the Porta Ardeatina left the planners only two options: to re- fortify all of the Aurelian line, or to give up all hope of modern fortifications for left-bank Rome. This was a weak position from which to face the inevitable problem of mounting costs. Altogether the cost of the works at the Porta Ardeatina and on theAventine was 44 000 scudi, a figure used by both Guglielmotti and Rocchi to explain the abandonment of the project [13]. Rocchi estimated the cost of Sangallo's entire project to be in the order of 460 000 scudi which was, in his opinion, too much for the finances of a 'small state'. Yet Sixtus V spent 255 000 scudi on the repair of the Aqua Felice viaduct, 90 000 on his chapel in S M Maggiore and 38 000 on the obelisk in front of St Peter's—altogether the ruler of this 'small state' invested more than one million scudi in civic improvements during his five-year pontificate [14]. Such comparisons, of course, take no account of inflation, nor do they include important extras such as the cost of the artillery needed to arm modern bastions. They do, however, make the point that costs alone often tend to be convenient rather than complete explanations for planning decisions. Cost conscious planners trim their schemes; yet when Sangallo's project went into the ground it was already much larger than the sketch scheme. If costs were indeed a major factor in the abandonment, the project was doomed when the planners turned their backs on the possibility of large-scale economies. SANGALLO'S DEFENCE OF ROME

Michelangelo's Porta Pia. There are many gaps in the history of Sangallo's Roman fortifications and many questions that will remain unanswered in the absence of primary sources such as the minutes of design meetings, which are available for some other sixteenth-century schemes. Our evidence has been largely circumstantial. The picture that emerges is one of a clash between military and civilian planning interests which will certainly be confusing to historians brought up to regard Renais- sance military architecture as a closed geometrical system, arbitrarily applied to towns. Lewis Mumford provides an extreme formulation of this view when he says that the 'cannon and the standing army... shifted the emphasis in building from architecture to engineering, from aesthetic design to material calculations' and reduced the city itself to no more than a 'mere appendage to the military form' [15]. The story of Sangallo's project suggests, on the contrary, that a purely military plan was changed, diluted and finally abandoned in response to economic and urban planning factors. This finding is consistent with other recent work. John Hale's study of the fortification of early seventeenth-century Vicenza showed that the political overtones of new works and their implications in terms of urban growth provoked lively debate both within the citizen body as well as between Venice and its subject city [16]. Horst De la Croix's account of the late sixteenth-century foundation of Palmanova demonstrated that the layout of an entirely new fortress-city involved compromise between the functional demands of the military engineers and the aesthetic requirements of the civilian architects [17]. Moreover, the writings of sixteenth- century theorists make it clear that rigidly applied geometrical schemes were for flat virgin sites — not for the fortification of existing cities. SIMON PEPPER

We are left with a general feeling of disbelief that non-military factors could be allowed to prejudice the security of towns in a dangerous age. People who had survived the horrors of sack, it will be objected, would be unlikely to put themselves at risk again by diminishing the effectiveness of their fortifications. There is, however, a great deal of evidence to suggest that society was no more willing to pay for defence in the sixteenth century than the twentieth century. Walls were neglected, improvements delayed, and modern fortifications often spoilt by illegal buildings blocking the fields of fire of the defending artillery [18]. Paradoxically, indifference and inertia could be combined with more positive attitudes: the conservative citizenry frequently expressed a genuine attachment to their ancient, and often useless, walls. When the walls of Florence were modernised in 1526-27; Benedetto Varchi, the Republican historian, was deeply distressed at the loss of 'the towers which, in the guise of a garland, crowned the walls' [19]. Princes, on the other hand, tended to invest in fortresses which were regarded, depending on the politics of the observer, as symbols either of tyranny or of the smack of firm government [20], Fortresses dominated Milan, Naples, Florence and Sienna. Paul III himself imposed fortresses in humiliating circumstances on the rebel cities of Perugia and Ascoli and, following the abandon- ment of his plan to refortify the Aurelian circuit, transformed the Roman Borgo into one of the most formidable armed camps in Europe. When Michelangelo built the Porta Pia in the Aurelian wall it was, as Ackerman observed, a piece of street scenery set in a wall which since 1542, had served a symbolic rather than a defensive function [21]. Perhaps this was the proper role for the walls 'garlanding' the capital city of the Christian world?

Arms of Nicholas V on repaired section of Aurelian wall. SANGALLO'S DEFENCE OF ROME

Notes 1. Ian A. Richmond, The City Wall of Imperial Rome ',in Technology and Culture, IV, 1963, , Oxford 1930. pp. 30-50. 2. Alberto Guglielmotti, Storia della fortificazioni 11. For Roman topography and urban develop- nella spiaggia Romana, Rome 1887; H. M. A. De ment see: Torgil Magnusson, Studies in la Croix, 'Military Architecture and the Radial Roman Quattrocento Architecture, Stockholm City Plan in Sixteenth-Century Italy', in The Art 1958; Jean Delumeau, Vie economique et Bulletin, vol XLII, no 4,1960, pp. 263-290 (see sociale de Rome dans la seconde moitie du pp. 277-78); Mario Borgatti, 'II bastione ardeatino XVIe siecle, 2 vols, Paris 1957. a Roma', in Rivista d'artiglieria e genio, 1916, ii, 12. Judith Hook, The Sack of Rome 1527, pp. 207-23; Cesare Quarenghi, Le mura di Roma, London 1972. Rome 1880. Guglielmotti and De la Croix name 13. Rocchi, op cit, vol I, pp. 239-40 gives a total the military and architectural consultants for the expenditure of 44000 scudi, some of which Roman project. Sangallo's extensive experience may be linked to the three individual sites: of military architecture is indicated by his works at the Porta Ardeatina, 17607 scudi; principal biographers, Gustavo Giovannoni, works on the Northern Aventine, 4735 scudi; Antonio da Sangallo il Giovane, 2 vols, Rome works on the Southern Aventine, 3446 scudi. 1959; A. Venturi, Storia dell'arte Italiana, Milan 14. Delumeau, op cit, vol I, passim. 1901-18, vol IX; G. Clausse, Us Sangallo, Paris 15. The City in Hislory, New York 1961, chapter 1902. This aspect of his work, however, still 12, sections 5 and 6. awaits detailed separate treatment. 16. J. R. Hale, 'Francesco Tensini and the 3. Della Architettura Militare, libri tre, Brescia Fortification of Vicenza', in Studi Veneziani, 1599. X, 1968, pp. 231-89. 4. Guglielmotti, op Cit, vol 5, pp. 322-29. 17. H. M. A. De la Croix, 'Palmanova: A study in 5. Op cit, p. 278. Sixteenth-Century Urbanism', Saggi e 6. Vinceneo Scamozzi, Dell'idea della architettura memorie di storia dell'arte, 1967, pp. 25-41. universale, Venice 1615, Book II, chap 28, p. 108. 18. As Marshal Vauban observed, 'It is common 7. Enrico Rocchi, Le piante iconografiche e in peacetime to see a city's inhabitants defile prospettiche di Roma del secolo XVI, 2 vols, the surroundings of the defences of a fortress Rome and Turin 1902. and change into a playground the area that 8. Vincent Scully Jr, 'Michelangelo's Fortification should serve for their fortifications'. Memoire, Drawings: A study in the Reflex Diagonal', Leiden 1740. The frequent promulgation of Perspecta, I, pp. 38-45. stern edicts against such activities are 9. The archival data is reproduced in an appendix to evidence of continued infringements, as much Rocchi, op cit, p. 225 ff. as their remedy. 10. The principles of renaissance fortification are well 19. Storia Fiorentina, Book II, chapter XXI. described in the following English language 20. J. R. Hale, 'The End of Florentine Liberty: works: F. L. Taylor, The Art of War in Italy, 1495- The Fortezza da Basso', in Florentine Studies, 1529, Cambridge 1921; Quentin Hughes, Military 1970, pp. 501-32; S. M. Pepper, 'The Architecture, London 1974; J. R. Hale, 'The Early Meaning of the Renaissance Fortress' in Development of the Bastion: An Italian Architectural Association Quarterly, vol 5, no Chronojogy cl450-cl534' in Europe in the Late 2, 1973, pp. 21-27. Middle Ages Ed Highfield, Hale & Smalley, 21. James Ackerman, Architecture of London 1965, pp. 466-94; De la Croix, op cit; and Michelangelo, London and New York 1961, 'The literature on Fortification in Renaissance chapter 10.