The four criteria for assessing class importance apply to bastles as follows:
- Period (currency): Transient. Examples are difficult to date within the bastle-building tradition, but the duration of the tradition was short, probably only 100 to 150 years. Although some bastles continued in use during post-medieval and modern times there is no evidence for the construction of new examples after about 1700.
- Rarity: Rare. On a national scale bastles represent an extremely scarce class of monument being confined in their distribution to the border country in northern England. Approximately 80 sites are known.
- Diversity (form): Low. All bastles share many characteristic features of design and layout, and display a restricted range of architectural features. The differences between barrel-vaulted and timber-floored types are fairly slight.
- Period (representativity): Low. Bastles are just one of a wide range of late medieval and post-medieval monument classes known, and are contemporary with many other classes of settlement monuments.
Assigning scores to these criteria following the system set out in the Monument Evaluation Manual, bastles yield a Class Importance Value of 12. This lies towards the bottom end of the range of class importance values (max.= 64). In selecting bastles of national importance, notice should be taken of the two types represented and the regional spread across northern England.