Several early stone keeps had been built after the conquest, with somewhere between ten and fifteen in existence by 1100, and more followed in the 12th century until around 100 had been built by 1216. Typically these were four sided designs with the corners reinforced by pilaster buttresses. Keeps were up to four storeys high, with the entrance on the first storey to prevent the door from being easily broken down. The strength of the design typically came from the thickness of the walls: usually made of rag-stone, as in the case of Dover Castle, these walls could be up to thick. The larger keeps were subdivided by an internal wall while the smaller versions, such as that at Goodrich, had a single, slightly cramped chamber on each floor. Stone keeps required skilled craftsmen to build them; unlike unfree labour or serfs, these men had to be paid and stone keeps were therefore expensive. They were also relatively slow to erect – a keep's walls could usually only be raised by a maximum of a year, the keep at Scarborough was typical in taking ten years to build.