In the early 12th century, Henry I began the construction of a stone keep at Corfe. Progressing at a rate of per year for the best part of a decade, the work was complete by 1105. The chalk of the hill Corfe Castle was built on was an unsuitable building material, and instead Purbeck limestone quarried a few miles away was used. By the reign of King Stephen (1135–1154) Corfe Castle was already a strong fortress with a keep and inner enclosure, both built in stone. In 1139, during the civil war of Stephen's reign, Corfe withstood a siege by the king. It is thought that he built a siege castle to facilitate the siege and that a series of earthwork about south-southwest of Corfe Castle mark the site of the fortification.