The standard Norman castle of the 11th century was the 'motte & bailey' - the motte being the earthen mound surmounted by the donjon or keep tower, the bailey being the surrounding enclosure and pallisade. The city of Oxford still retains its old Norman castle mound, and I have always wanted to get an Arts Council grant to install a Bouncy Castle on top and enter it for the Turner Prize...
The Motte & Bailey castle was a Norman idea. At the invasion in 1066, the Normans erected three of these castles. The Motte was an earth mound made from the earth spoil from the ditch which was dug by the local "volunteers". The Normans then erected the wooden castle on top of the Motte. This enabled them to dominate the locality. During the following years hundreds of these "prefab" castles were built and when the site proved efective they replaced the wooden structure with stone.
Cardif Castle is a very good example of the finished product