With both vision and resourcefulness, and by exploiting Welsh
          tradition and adapting feudal habits, he tried to create a powerful
          and permanent principality. It took a struggle, from 1188 till 1203,
          before he mastered Gwynedd.
               In 1205, by marrying king John's illegitimate daughter, Joan, he
          hoped to keep the peace with England. However, his own aggression in
          Powys made King John retaliate and Llewelyn allied with the French.
          Taking advantage of the civil war in England, he extended his dominion
          into Carmarthenshire and Cardiganshire. In 1216 he made himself Lord
          of Powys which, in 1218, was acknowledged by the English.
              In 1229, against Welsh tradition, he tried to secure recognition
          for his son Dafydd as his successor; by which he snubbed his elder
          bastard son Gruffydd, a headstrong man who was deeply resentful. Then
          in 1230 he hanged William de Braose, his heir's father-in-law, for
          having had an affair with Llewelyn's wife.
              Also in 1230 he took the unique title of Prince of Aberffraw and
          Lord of Snowdon which, it was claimed, implied superiority over other
          Welsh rulers. However, he was forced to pay homage to Henry III, King
          of England. In 1237 he possibly had a stroke and it was his son,
          Dafydd II, who probably governed until 1240 when Llewellyn died.
               He was buried in his favourite monastery, Aberconwy, where he
          joined his wife who had died in 1237.

Source: Leo van de Pas