motte-and-bailey castles were converted to stone, with the keep amongst usually the first parts to be upgraded, while in other cases new keeps were built from scratch in stone. These stone keeps were introduced into Ireland during the 1170s following the Norman occupation of the east of the country, where they were particularly popular amongst the new Anglo-Norman lords. Two broad types of design emerged across France and England during the period: four-sided stone keeps, known as Norman keeps or great keeps in English – a donjon carré or donjon roman in French – and circular shell keeps. The reasons for the transition from timber to stone keeps are unclear, and the process was slow and uneven, taking many years to take effect across the various regions. Traditionally it was believed that stone keeps had been adopted because of the cruder nature of wooden buildings, the limited lifespan of wooden fortifications and their vulnerability to fire, but recent archaeological studies have shown that many wooden castles were as robust and as sophisticated as their stone equivalents. Some wooden keeps were not converted into stone for many years and were instead expanded in wood, such as at Hen Domen. Nonetheless, stone became increasingly popular as a building material for keeps for both military and symbolic reasons. Stone keep construction required skilled craftsmen. Unlike timber and earthworks, which could be built using unfree labour or serfs, these craftsmen had to be paid and stone keeps were therefore expensive. They were also relatively slow to erect, due to the limitations of the lime mortar used during the period – a keep's walls could usually be raised by a maximum of only 12 feet (3.6 metres) a year; the keep at Scarborough was not atypical in taking ten years to build. The number of such keeps remained relatively low: in England, for example, although several early stone keeps had been built after the conquest, there were only somewhere between ten and fifteen in existence by 1100, and only around a hundred had been built by 1216. Norman keeps had four sides, with the corners reinforced by pilaster buttresses; some keeps, particularly in Normandy and France, had a barlongue design, being rectangular in plan with their length twice their width, while others, particularly in England, formed a square. These keeps could be up to four storeys high, with the entrance placed on the first storey