A cliff castle is a coastal promontory adapted as an enclosure by the construction of one or more ramparts placed across the neck of a spur, in order to divide it from the mainland. The promontories chosen usually end in steep cliffs on the seaward side. The continuation of the ramparts along the edges of such bluffs is extremely unusual. Where ramparts do occur on the seaward side it is generally on sites where the land slopes gradually down to the beach.
The boundaries are formed by banks or walls, accompanied by external ditches. They are usually broken by a single entrance, marked by a simple gap in the rampart or ramparts. On cliff castles with multiple boundaries the entrance through each rampart is generally in alignment, although staggered examples are known.
The area enclosed varies according to the size and shape of the promontory chosen. Internal features are infrequent and are found mainly in the form of hut circles or platforms, used to provide level ground for buildings of circular construction.
Cliff castles should be distinguished from other classes of hillfort which occur near to the coastline. Unlike these related monument classes, cliff castles will always be located on promontories above the sea or riverine estuaries with ramparts which separate those promontories from the mainland.
Cliff castles are usually interpreted as defensive enclosures and are thought to represent a class of hillfort. Some were certainly occupied, but the extent to which this is true of all sites is uncertain.