A motte and bailey castle comprises a large conical or pyramidal mound of soil or stone (the motte) surrounded by, or adjacent to, one or more embanked enclosures (the bailey). Both may be surrounded by wet or dry ditches and could be further strengthened with palisades, revetments, and/or a tower on top of the motte.

The motte and the bailey of a motte and bailey castle need not have been contemporary in origin. Sometimes a motte could be added to an existing defensive enclosure, while in other cases a bailey was added to an isolated motte. For the purposes of evaluation, isolated mottes are specifically excluded from the definition of motte and bailey castle and are considered as a separate class. Also excluded are defended enclosures without mottes; these, defined as ringworks, are considered as a separate class too.

Motte and bailey castles are usually fairly easy to identify as field monuments, although sometimes large round barrows, windmill mounds, garden landscape features, and isolated mottes have been confused with the mottes of motte and bailey castles. It is possible that some mounds may have been used for more than one of these purposes; the siting of the mound can be significant as the motte and bailey castles may dominate a road, river crossing, or settlement; the presence of a bailey is conclusive. Close attention to historical documents can be an important factor in recognising and authenticating motte and bailey castles.

Motte and bailey castles were military strongholds, built as a base for offensive operations, and are found in urban areas and in rural settings.