 As part of his strategy for dealing with the Marcher Lords , Llywelyn married his eldest son , Dafydd , to Isabella , the daughter of William de Braose , a powerful lord in Brecon , Builth and Abergavenny . The Marcher Lords had adopted a style of stone castle that included circular keeps and an integrated system of curtain walls . Following Dafydd 's marriage , Llywelyn appears to have started a second phase of building at Dolbadarn , probably in the 1240s , adding these elements to the existing castle . The prince was probably aiming not only to incorporate the latest military technology , but also to create a castle of equal prestige to those of his new allies in the south . Traditionally the surrounding district of Is Gwyrfai had been run from the town of Llanbeblig ; after the 1240s , this role was transferred to Dolbadarn . 
