The coastal version is familiar enough, a headland cut off from the mainland by a ditch or rampart, using the sea cliffs as natural defences on three sides. The inland variety works on the same principle but substitutes a river bend, a bog, or a lake peninsula for the ocean edge. Whoever built the fort at Errew was doing something conceptually identical to their coastal counterparts, simply with different water or terrain doing the work of the cliffs.

Promontory forts of this kind are generally associated with the Iron Age, though many continued in use or were adapted across later centuries. The defining feature is economy of effort: nature provides most of the defensive barrier, and the builders need only close off the narrow neck of land where the promontory meets the wider ground. A single bank and ditch across that neck could render a naturally defended spit of land into a secure enclosure. Errew sits in a landscape shaped by Lough Conn and its surrounding wetlands, an area where peninsulas and marginal land between water and bog would have offered precisely the kind of topography that made this form of fortification practical. Beyond its classification and location, the detailed history of this particular site remains to be fully documented.