From the early 12th century onwards the Normans began to build new castles in stone and convert existing timber designs. This was initially a slow process, picking up speed towards the second half of the century. Traditionally this transition was believed to have been driven by the more crude nature of wooden fortifications, the limited life of timber in wooden castles and its vulnerability to fire; recent archaeological studies have however shown that many wooden castles were as robust and as complex as their stone equivalents. Some wooden castles were not converted into stone for many years and instead expanded in wood, such as at Hen Domen.