The motte is a raised earth mound, usually artificial and topped with a wooden or stone structure. The earth for the mound would be taken from a ditch, dug around the motte or around the whole castle. The outer surface of the mound could be covered with clay or strengthened with wooden supports.
The bailey is an enclosed courtyard, typically surrounded by a wooden fence and overlooked by the motte. A castle could have more than one bailey, sometimes an inner and an outer.
Motte-and-bailey castles could be very quickly erected; according to records, William the Conqueror built one at Pevensey in eight days. The rapidity and ease with which it was possible to construct castles of this type made them characteristic of the Norman Conquest period in England and of the Anglo-Norman settlements in Wales, Ireland and the Scottish lowlands. In later days a stone wall replaced the timber palisade and produced what is known as the shell-keep, the type met with in the castles of Berkeley, Alnwick and Windsor, still existing today. The remains of castle mottes can be found in many parts of Britiain today.
A description of this type of castle is given in the life of John, bishop of Terouanne (Ada Sanctorum, quoted by GT Clark, Medieval Mil. Architecture): "The rich and the noble of that region being much given to feuds and bloodshed, fortify themselves ... and by these strongholds subdue their equals and oppress their inferiors. They heap up a mound as high as they are able, and dig round it as broad a ditch as they can ... Round the summit of the mound they construct a palisade of timber to act as a wall. Inside the palisade they erect a house, or rather a citadel, which looks down on the whole neighbourhood". St John, bishop of Terouanne, died in 1130, and this castle of Merchem, built by a lord of the town many years before, may be taken as typical of the practice of the 11th century. But in addition to the mound, the citadel of the fortress, there was usually appended to it a bailey or basecourt (and sometimes two) of semilunar or horseshoe shape, so that the mound stood on the line of the enceinte.