Building was started in 1067 by Earl William fitz Osbern, close friend of William the Conqueror, making it one of the first Norman strongholds in Wales.
Dechreuwyd y gwaith adeiladu ym 1067 gan Iarll William fitz Osbern, cyfaill agos i Gwilym Goncwerwr, gan ei wneud yn un o’r cadarnleoedd Normanaidd cyntaf yng Nghymru.