"I was the cradle of Welsh Kings long before the age of the castle, although my crowning achievement is something of a folly. Two of my Welsh brothers live nearby. What castle am I and can you explain my riddle?"
Yes, the correct answer is DINEFWR CASTLE!
Dinefwr was the seat of power for the pre-medieval Welsh dynasty of Deheubarth, and as such is associated with some of the most important early rulers of Wales, including Rhodri Mawr, Hywel Dda, and later Rhys ap Tudor, and Rhys ap Gruffydd. Parts of the castle were still in use in the post medieval period, and Dinefwr's final construction project was the placement of a "folly-summer house" atop the castle's great keep. Carreg Cennen and Dryslwyn, Welsh castles, that, like Dinefwr, were built by the native princes of Wales are situated close by.