The idea behind a coastal promontory fort, known in Irish archaeology as a dún or ráth in its inland form, was elegantly practical: let the sea do most of the defensive work. A headland naturally protected on two or three sides by cliffs and water needed only a rampart, or sometimes a series of them, thrown across the neck of land to create an enclosure that was genuinely difficult to approach. The result was a form of fortification found all along Ireland's Atlantic edge, and the Castlehaven example is one of many such sites on the Cork coastline where the engineering and the geography still speak to each other across several millennia.

Castlehaven itself, a sheltered harbour inlet near Skibbereen, sits in a part of Cork that has been inhabited, contested, and observed from the sea for a very long time. The harbour's deep water made it strategically significant well into the early modern period, most notably during the landing of Spanish forces in 1601 ahead of the Battle of Kinsale. The choice of such inlets for early settlement was rarely accidental, and promontory forts along this stretch were almost certainly built to watch and control the approaches that later generations found equally useful. These structures are generally dated to the Iron Age, roughly 500 BC to 400 AD, though some remained in use or were adapted considerably later.