On the southern flank of Cader Idris is the 13th century Castell Y Bere. Unlike the more famous "ring" of English casles built around the area in the late 13th century this one was built by the Welsh to keep the English out. It didn't work; as Carol said at the time we visited it, " why build it here, you could just walk around it...." a very scenic walk it would be too :-)
Compare with Harlech Castle just a few miles away and about the same age, Harlech has the building structure but Castell Y Bere has the better scenery.