The Ordnance Survey mapped that arch as a "Cavern" on both editions of its six-inch sheet, a small cartographic curiosity that hints at how unusual this particular piece of coastline is. What the maps do not immediately convey is that this promontory was also, at some point in prehistory, turned into a defended enclosure.

A promontory fort works on a straightforward principle: the sea does most of the work, and human effort is concentrated on the landward approach. At Lackavaun, two banks cut across the neck of the promontory, one behind the other. The outer line combines a natural rock outcrop on its western side with an earthen bank rising about half a metre. Behind it, the inner bank stands roughly a metre tall and has been faced with stone on its outer edge, a detail that suggests some care in construction even if the overall scale is modest. The defended interior is not a smooth plateau but a more dramatic, irregular space, two rocky ridges bound together at their seaward end by that arched passage through the rock. Whether the fort's original occupants used that arch as a sheltered landing point, a dramatic threshold, or simply regarded it as a curiosity of their own, the notes do not say.