Traeth Mawr and Traeth Bach, two rock hollows towards the south-western end of the common are now filled with peat, core sections of which have proved valuable in unravelling the climatic and vegetational history of this area over the last 10,000 years.
The Brecon Beacons Mountain Centre stands next to Mynydd Illtud on its eastern side and provides advice for visitors wishing to enjoy the area.
Though little remains of the ancient structure, the location affords good views towards the peaks of Pen-y-Fan and Corn Du as well as to the northern slopes of the Black Mountains and the river Usk.
These excellent views of the surrounding area may explain why its naturally defensive spot was chosen for a settlement.
Near Twyn y Gaer are a few pillow mounds which date from the 18th and 19th centuries, when rabbits were bred on a large scale for their meat and fur.