The promontory extends from the coastline in the rough outline of a boot, 102 metres long and up to 56 metres wide, and it belongs to a category of prehistoric monument found all along the Atlantic seaboard of Ireland. A promontory fort works on a straightforward principle: where the sea protects three sides of a headland, you only need to fortify the fourth. What makes Tonalassa worth attention is how much of that fortification survives, and how it has accumulated layers from different centuries.

The landward side of the promontory is defended by a substantial bank, stone-faced and rising to a maximum of 5 metres above the fosse in front of it. A fosse is essentially a defensive ditch, and this one runs for 65 metres with an average width of 5 metres, crossed by a narrow causeway of around 2 metres. The antiquarian T. J. Westropp, writing in 1909, recorded that there had been an outer bank as well, but it had already been completely levelled by his time, with its material slumping into and partly filling the ditch. The entrance gap through the main bank, about 8 metres wide, sits roughly 13 metres from the northern cliff edge. It is here that the site takes an unexpected turn: just inside the gap, on the left as you enter, are the remains of a small tower constructed during the First World War as a coastal lookout. A prehistoric enclosure repurposed as a military observation post is a fairly compressed illustration of how Kerry's coastline has always mattered strategically. A small rath, another form of enclosed settlement, lies to the south-south-east of the promontory, suggesting this corner of the headland was a focus of activity across multiple periods rather than a single isolated episode of building.