Hen Gaer is an Iron Age hillfort, on a hill adjacent to and east of Bow Street, Ceredigion, Wales.

[1] Hen Gaer ("Old Castle") is smaller than a typical tribal fortification.

On the north-east side, carefully laid stone blocks of the original rampart wall can be seen.

There is a circular mound outside the entrance: it is not an earlier barrow, but may be a defensive feature.

[1][2] The enclosed area includes some of the southern hillslope; it is thought this may have been to make the fortification visible from the narrow valley below, at the confluence of Afon Stewi and Nant Seilo forming Afon Clarach.