That last detail is what gives the site at Tomdarragh its particular quality: despite being a substantial construction, measuring roughly 102 metres north to south and 98 metres east to west, no entrance has ever been identified. Local tradition, however, insists that a passage leads into the site somewhere, a detail that has persisted without any corresponding physical evidence to confirm it.

The enclosure is defined by a double-fosse arrangement, that is, two ditches separated by a bank, a design that suggests the site was intended to be both bounded and defended, or at least clearly demarcated. The inner fosse runs four to five metres wide, the earthen bank between them three to four metres across, and the outer fosse widens to between five and seven metres. Towards the south, where the natural hillock reaches its highest point, the outer fosse gives way to a broad berm, a flat ledge cut as a step into the natural slope, adding a further layer of shaping to the terrain. At that same southern high point, the foundations of walls forming an angle are visible, the only structural remains above ground. It is not known what building they once belonged to, or how they relate to the enclosure itself.