to prevent the door from being easily broken down; early French keeps had external stairs in wood, whilst later castles in both France and England built them in stone. In some cases the entrance stairs were protected by additional walls and a door, producing a forebuilding. The strength of the Norman design typically came from the thickness of the keep's walls: usually made of rag-stone, these could be up to 24 feet (7.3 metres) thick, immensely strong, and producing a steady temperature inside the building throughout summer and winter. The larger keeps were subdivided by an internal wall while the smaller versions had a single, slightly cramped chamber on each floor. Usually only the first floor would be vaulted in stone, with the higher storeys supported with timbers. There has been extensive academic discussion of the extent to which Norman keeps were designed with a military or political function in mind, particularly in England. Earlier analyses of Norman keeps focused on their military design, and historians such as R. Brown Cathcart King proposed that square keeps were adopted because of their military superiority over timber keeps. Most of these Norman keeps were certainly extremely physically robust, even though the characteristic pilaster buttresses added little real architectural strength to the design. Many of the weaknesses inherent to their design were irrelevant during the early part of their history. The corners of square keeps were theoretically vulnerable to siege engines and galleried mining, but before the introduction of the trebuchet at the end of the 12th century, early artillery stood little practical chance of damaging the keeps, and galleried mining was rarely practised. Similarly, the corners of a square keep created dead space that defenders could not fire at, but missile fire in castle sieges was less important until the introduction of the crossbow in the middle of the 12th century, when arrowslits began to be introduced. Nonetheless, many stone Norman keeps made considerable compromises to military utility. Norwich Castle, for example, included elaborate blind arcading on the outside of the building and appears to have had an entrance route designed for public ceremony, rather than for defence. The interior of the keep at Hedingham could certainly have hosted impressive ceremonies and events, but contained numerous flaws from a military perspective. Important early English and Welsh keeps such as the White Tower, Colchester, and Chepstow were all built in a distinctive Romanesque