Dinas Dinorwig hillfort sits on one of the summits of a long ridge running NE-SW. On the NW the ground falls away steeply to a small tributary of the River Seiont and on the SE the ground falls away less steeply to the marshy valley in which the River Cegin rises.

The fort consists of an inner wall, enclosing circa one hectare, surrounded by two massive ramparts of earth and rubble. The larger of the two entrances through the wall is obstructed by the inner rampart, showing that the ramparts are later than the wall. The outer rampart is accompanied for most of its circuit by a ditch some four meters wide. On the North side a wedge shaped annexe occupies the space between the outer rampart and the steep slope of the hill. Within the defences, on the west side, lies a single hut, now visible as a depression.
Source: Caernarvonshire Inventory, Vol II, No 1170. RCAHMW.