Craig-y-Dinas, which crowns a prominent outcrop, is a stone walled polygonal hillfort measuring some 90m NE/SW by 40m NW/SE externally, with outworks extending 14m to the north-east. The fort encloses 0.18 hectare. It has extensive entrance works to the east where stone clearance and walling for an approach trackway can be traced for about 80-100m. There is a prehistoric roundhouse settlement below the entrance to the east and a farmstead of post medieval character just to the south.
The hillfort is first mentioned by Thomas Pennant in his ‘A Tour in Wales’ (The Journey to Snowdon: MDCCLXXXI. (1781)), as he stayed at Cors y Gedol hall nearby. He noted
‘I first visited Craig y Dinas, the summit of a hill, surrounded with a vast heap of stones, the ruins of a wall, which, in many parts, retain a regular and even facing: this, and some others similar, are the first deviations from the rude ramparts of stone, and prior to the improvement of masonry by the use of mortar. Into this is an oblique entrance, with stone facings on both sides; and near it are two ramparts of stones. The whole is on the steep extremity of the hill, near to which is a pass into the country.’