As time wore and siege machines developed, castle fortifications were forced to evolve once again in order to keep up with the evolving military technology. One important factor in the castle’s evolution away from the rectangular keeps was the crusades (Gies 27). The Western Crusaders had, by the end of the First Crusades, “secured their presence in the Holy Land” (DeVries & Smith 234). However, their claimed lands were threatened as time wore on, and the crusaders were faced with the unenviable position of reasserting their claim “with fewer soldiers than they had in the initial conquests” (DeVries & Smith 234). There were two methodologies: negotiation with the locals and castle-building; the second proved to be more reliable (DeVries & Smith 234). It became clear that the “Crusaders needed a fortification that was larger, more quickly built, and more defensible as it was being built” (DeVries & Smith 236), so they intimidated the local “older Byzantine fortresses,” a style that is described as a castle complex or a concentric castle (DeVries & Smith 236-237).
In addition to exposing them to new forms of castle construction, the crusades exposed the crusaders to the East, which “introduced to the Western world efficient methods of siege warfare and sophisticated defensive techniques” (Lepage 72). As the crusaders marched home, they brought with them knowledge that transformed siege warfare in their own lands (Toy 90). As a result, castle planning adopted a “more scientific plan” intended to counteract these newly learned siege tactics (Toy 116). This contributed “to the full development of medieval military architecture in the 13th century and its brilliant apogee in the 14th” (Lepage 91).
However, despite the Crusaders learning these tactics in the early twelfth century, back home the transition from the rectangular keep to the circular keep took place slowly (Gies 25). After all, rectangular keeps were still being built “by Henry II in his numerous late-twelfth- and early-thirteenth-century castle-building projects” (DeVries & Smith 233). One explanation for this gradual transition lies in the hassle that the circular keeps posed to its inhabitants (Gies 25). For example, the circular interiors resulting from the circular design were perceived as being “less practical for daily life than rectangular ones” (Lepage 63). For a time, there was some investigation into constructing “keeps that were circular on the outside and square on the inside” (Gies 25). For the most part, the transitional keeps that were built tried to maintain “the advantages of both forms” (Toy 90).
However, shortly after “the third Crusade at the end of the twelfth century” builders seem to have admitted defeat and accepted the round keep, with all its advantages, as the typical structure (Toy 90). This acceptance of the circular keep was likely fueled by new developments in siege warfare. The early thirteenth century supposedly saw the development of the trebuchet (Jones 174), which was “probably introduced during the Crusades” (Lepage 96). It seems to be difficult for historians to date the trebuchet exactly (Bradbury 259) due to the “casual” use of terms by medieval chroniclers (Bradbury 261), but for the most part “evidence from the twelfth century provides a good case for its introduction then, although one has to wait for the thirteenth century for conclusive proof” (Bradbury 260).
The trebuchet had counterweights and was both more “powerful” and more “accurate” than any of the previous siege engines (Jones 174). This new machine definitely gave the attackers the advantage (Jones 174-175). As a structure with defense as its primary function, the castle naturally had to evolve to counteract this new threat. The design shifted because the “number of flat surfaces prone to such bombardment” had to be decreased (Jones 174). In other words, the keeps were rounded, and even the outer wall’s “square flanking towers were replaced by semi-circular or convex mural towers” (Jones 175). An example of this new style of keep can be seen in Windsor Castle. The tower did undergo renovations later, so keep in mind that it has been updated, but it is still a good example of a circular keep.
There was also a slight change in the keep’s purpose. Previously the keep had been designated as the lord’s living space, but in the newer style all “the living quarter…were now all built in the court of the inner bailey” (Toy 116). This new style of keep was still intended as a last line of defense, but now it was not necessarily the lord’s primary residence (Toy 116). It was also “smaller than those built previously but of a more powerful and scientific design” (Toy 116). Surprisingly, if a rectangular keep had already been constructed, it was that immediately abandoned (Toy 116). Instead, “one or two outer baileys were added on the line of approach” (Toy 116). This is demonstrated by additions that were later added to Dover Castle, which is depicted in the image below. You can see the square keep surrounded by outer castle complex structures. The circular towers on the outer walls over the gate (called Constable’s Gate) were definitely added later (Brindle 53).
Ultimately the English rectangular keep and its place in castle architecture’s evolution can be best summarized by a quote from Joseph and Frances Gies in the first chapter of their book Life in a Medieval Castle:
Thus the castle, born in tenth-century continental Europe as a private fortress of timber and earthwork, brought to England by the Normans, converted to stone in the shell keeps and rectangular keeps of the eleventh and twelfth centuries, refined and improved by engineering knowledge from Crusading Syria, achieved its ultimate development at the end of the thirteenth century in the western wilds of the island of Britain. (Gies 31)
“19th Century French Three-Quarters View Drawing of a Medieval Counterweight Trebuchet.” Dictionnaire Raisonne De L’Architecture Francaise Du Xle Au XVe Siecle. 1854-1868. Wikipedia, 15 May 2006. upload.wikimedia.org/wikipedia/en/6/63/Trebuchet1.png. Accessed 25 Nov. 2016.
Bradbury, Jim. “Chapter 9: Medieval Siege Weapons.” The Medieval Siege. The Boydell Press, 1992.
Brindle, Steven. Dover Castle. Edited by Katy Carter. English Heritage Guidebooks, 2012.
chaoticblackcat. “Dover Castle From Outside the Gate.” 14 July 2014.
DeVries, Kelly, and Robert Douglas Smith. “Chapter Nine: Stone Castles.” Medieval Military Technology, 2nd ed., University of Toronto Press, 2002. pp. 223-260.
Giel, Immanuel. “The Round Tower in the Middle Ward, Built by Henry II and Remodeled in the 19th Century.” Wikipedia, 07 Aug. 2006. commons.wikimedia.org/wiki/File:Windsor_round_tower_03.JPG. Accessed 03 Dec. 2016.
Gies, Joseph, and Frances Gies. “Chapter 1: The Castle Comes to England.” Life in a Medieval Castle. Perennial Library, 1974. pp. 8-31.
Jones, R.L.C. “Fortifications and Sieges in Western Europe c. 800-1450.” Medieval Warfare: A History, edited by Maurice Keen, Oxford University Press, 1999. pp. 163-185.
Lepage, Jean-Denis, G. G. “Chapter 2: The Revival of Military Architecture from the 10th to the 12th Centuries.” Castles and Fortified Cities of Medieval Europe: An Illustrated History. McFarland & Company, Inc., Publishers, 2002. pp. 28-106.
Toy, Sidney. “Chapter IX: Transitional Keeps of the Twelfth Century.” Castles: Their Construction and History. 1939. Dover Publications, Inc., 1985. pp. 90-99.
Toy, Sidney. “Chapter X: Fortifications and Buildings of the Bailey in the Eleventh and Twelfth Centuries.” Castles: Their Construction and History. 1939. Dover Publications, Inc., 1985. pp. 100-115.