The principle was straightforward: choose a finger of land where cliffs or steep slopes guard three sides, then cut a bank and ditch across the narrow neck of the promontory to seal off the fourth. The result was a defended enclosure requiring far less labour than an inland ringfort, and one that could be abandoned quickly by boat if necessary. These structures are found at intervals all around the Irish coastline, and most are thought to date broadly to the Iron Age, though some were likely reused or adapted across many centuries.