Where the land narrows and drops sharply to the sea on three sides, the builders of such a fort needed only to cut off the landward approach, typically with a bank, ditch, or stone rampart, to create a naturally formidable enclosure. The sea itself became the wall.

Cill Éinne, whose name refers to the church of Saint Éinne, or Enda, sits at the eastern end of Inis Mór and carries considerable early medieval significance. Saint Enda founded a monastery here in the fifth or sixth century that became one of the most influential ecclesiastical sites in early Christian Ireland, drawing scholars and clerics from across the island and beyond. Promontory forts of this coastal type are generally associated with the Iron Age, though some remained in use or were adapted across later periods, and the concentration of such monuments along the western seaboard reflects both the strategic value of Atlantic headlands and the long human presence on these islands. The relationship between the fort at Cill Éinne and the later ecclesiastical landscape around it is the kind of layered occupation that makes the Aran Islands particularly dense with archaeology.