A promontory fort is exactly what the name suggests: a defensive enclosure built where a headland juts into the sea, with the water doing much of the work on three sides and an earthen or stone rampart closing off the landward approach. The result is a naturally fortified peninsula, requiring far less construction effort than an inland ringfort while offering a commanding view of the approaches by both land and sea.

These coastal forts are found at intervals along the Irish shoreline and are generally associated with the Iron Age, though some were constructed or reused across a broad span of prehistoric and early historic periods. The choice of a promontory was not simply defensive. Such positions also signalled status and authority over a stretch of coastline, controlling movement, fishing grounds, or landing places. At Castlepoint, the site carries that same logic, a place chosen because geography itself became architecture.