|Steve Edwards' website|
The motte-and-bailey was a key form of strategic infrastructure in the Normans' conquest of England and, a century later, in the Anglo-Norman partial-conquest of Ireland.
The motte-and-bailey was a type of proto-castle, built as an advance position in an offensive military campaign. The motte was a conical mound of earth topped by a wooden or stone military tower, the "keep." The bailey was a small living space encircling or adjacent to the motte.
The bailey, often separated from the motte by a removable bridge across a trench, was itself generally surrounded by a similar trench, and a mound of soil. A wooden-stake fence would stand atop the ring-mound.
The tower (or keep) upon the motte, of course, would serve as the last refuge during attack, and as a vantage from which to fight.
The tactic of building the motte-and-bailey was an important asset for the Normans in their invasion of England. They built the first one at Hastings, upon arrival. They built many William The Conqueror having declared himself king of all England with an army of 10,000 in a nation of a million or more.
It was after a territory had become suitably conquered that the Normans (and Anglo/Cambro/Flemish-Normans) began making the stone castles for which they are famous, which themselves served only a relatively brief military role.