The National Monument Record, available through Coflein describes Hen Gaer in quite a bit of detail:
Hen Gaer is a strong and imposing fortress, commanding panoramic views over the lowland basins north of the Rheidol, north to Caer Pwll Glas and beyond to the Dyfi Estuary. The fort is of massive construction. Carefully laid stone blocks of original rampart walling can be seen exposed on the northeast side. The rampart measures about 12m wide overall and still stands in places to a height of 3-4m (Hogg, Cardiganshire County History 1994, 264), with an outer rock-cut ditch (best visible on the northeast).
Hen Gaer is unusual in that part of the rampart encloses a considerable hillslope to the south, avoiding a more level summit position to the north, which must have made the construction of houses in the southern part very difficult. One possible explanation of this non-utilitarian siting is that the fort had a role to command the ridge, but also to exert a degree of influence over, and to remain highly visible from, the restricted lowland basin at the confluence of the Afon Stewi and Silo to the south.
The author of the above is Toby Driver, local hillfort expert.