The principle is straightforward: where the sea does most of the work, cutting away on three sides, the builders needed only to seal off the landward approach with a bank or ditch to create a defensible enclosure. Hundreds of these forts survive along the Irish coastline, and yet each one occupies its ground in a slightly different way, shaped by the particular geometry of the headland and the needs of whoever chose to settle or take refuge there.

Promontory forts are generally associated with the Iron Age, though some were built or reused considerably later, and without excavation it is rarely possible to say precisely when any individual example was occupied or for how long. The name Castlepoint itself is suggestive, hinting at a long local memory of something fortified and commanding at the tip of the land, even if the structure predates anything that would ordinarily be called a castle by many centuries. Cork's coastline is particularly well furnished with examples, a reflection both of its deeply indented geography and of the density of early settlement along its sheltered inlets and exposed headlands.