A tower keep castle is a strongly fortified residence in which the keep is the principal defensive feature; the keep may be freestanding or surrounded by a defended enclosure. They will be recognised in the field by the keep, a large stone tower,usually square although other shapes are known; they have several floors providing accommodation of various kinds. In documentary sources the class may be distinguished from other castle classes by references to the existence of a "donjon", commonly used as a synonym for the "great tower" or "tower keep". The main components are the keep, an external staircase, or a fore-building housing the stairs, and possibly a drawbridge. If an enclosure exists there will be a gatehouse, walls, and probably an external ditch, and there may be some buildings inside the enclosure.

Sites that may be confused with this class are shell keeps and quadrangular castles. A shell keep is a masonry enclosure on a motte or ringwork, possibly with "lean-to" buildings inside of one storey, whilst quadrangular castles, as the name implies, are rectangular or square and built around a quadrangle with the buildings seldom of more than two storeys.

Excluded from this monument description are enclosure castles with keeps, commonly arising where the continued development of that site has resulted in the principal defensive function of the keep being assumed by the enclosure walls and towers. Also excluded are the freestanding towers found mostly in the northern counties and commonly called "bastles" or "pele towers"; these are little more than defended farmsteads without the strategic function of a castle.

Tower keep castles were the strongly fortified residence of the king or a lord, sited for defensive or offensive operations, and often being an administrative centre; they are found in urban and rural areas. They were built throughout the medieval period from immediately after the Conquest, with a peak in the middle of the 12th century, to the mid-15th century, and many were developed into major castles of the enclosure class.