They could also be built as a mix of timber and stone, and sometimes stone buildings were built on existing mottes. When a stone wall replaced the timber palisade of the existing structures, it produced what is known as a "shell-keep"; the type met with in the extant castles of Berkeley, Alnwick and Windsor in England. In southern Europe stone castles became predominant from the mid-11th century, spread by the Norman conquests; the same occurred in the Holy Land through the Crusades, although Islamic and Byzantine influences were also present. In Germany, the equivalent of the keep was called the Bergfried.
The Normans introduced two other types of castle. The one was adopted where they found a natural rock stronghold which only needed adaptation, as at Clifford, Ludlow, the Peak and Exeter, to produce a citadel; the other was a type wholly distinct, the high rectangular tower of masonry, of which the Tower of London is the best-known example, though that of Colchester was probably constructed in the 11th century also. But the latter type belongs rather to the more settled conditions of the 12th century when haste was not a necessity, and in the first half of which the fine extant keeps of Hedingham and Rochester were erected. These towers were originally surrounded by palisades, usually on earthen ramparts, which were replaced later by stone walls. The whole fortress thus formed was styled a castle, but sometimes more precisely "tower and castle", the former being the citadel, and the latter the walled enclosure, which preserved more strictly the meaning of the Roman castellum.
Reliance was placed by the engineers of that time simply and solely on the inherent strength of the structure, the walls of which defied the battering ram, and could only be undermined at the cost of much time and labour, while the narrow apertures were constructed to exclude arrows or flaming brands.
In the 11th century fortification architecture was also prominent, and probably superior to its Western equivalent, in Islamic countries. Fortress there, when possible, took advantage of the terrain characteristics, and the walls were intervalled by flanking towers with, sometimes, a detached towers (albarrana). During the Spanish Reconquista, a keep (torre del homenaje, Tower of the homage) was added by the Christians when they captured these castles, as it happened for the castle of Banos in 1212.
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