The four criteria for assessing class importance apply to tower keep castles as follows:

Period (currency): Extended. The tradition of constructing tower keep castles lasted for about 350 years, starting just after the Conquest, with most being built in the 12th century, and the last in the mid 15th.

Rarity: Rare. A distinction to be noted is the division of this class into two types, the tower (55) and the hall (27); the latter type should be regarded as Very Rare. The numbers known may increase slightly as a result of excavations, but probably not significantly.

Diversity (form): Very high. The classification shown above shows the diversity of this class of monument; there are 2 basic forms, at least 5 shapes, 2 relationships with a ward if one is attached, and several combinations of important components.

Period (representativity): High. Although there are many classes of medieval monument tower keep castles is one of the few devoted to defence and use as an important residence.

Assigning scores to these criteria following the system set out in the Monument Evaluation Manual, tower keep castles yield a Class Importance Value of 46. This is nearly threequarters up the range of possible values (max.=64), reflecting the wide diversity of this class of monument. In selecting examples of national importance attention should be given to getting a mix of types and a range of sites in different situations.