 Neighbouring Welsh kingdoms and the English took advantage of this to infiltrate Deheubarth and it was around this time that Dryslwyn Castle was built.It is not clear precisely who built it, but it was probably constructed in the 1220s by one of the princes of Deheubarth, perhaps Rhys Gryg.In any event, the castle at Dryslwyn was, along with the neighbouring Dinefwr Castle, for a long time central to the security of the kingdom.