Gilbert de Clare, 6th Earl of Hertford, 7th Earl of Gloucester was a powerful English noble. He was also known as "Red" Gilbert de Clare or "The Red Earl", probably because of his hair colour or fiery temper in battle. He held the Lordship of Glamorgan which was one of the most powerful and wealthy of the Welsh Marcher Lordships as well as over 200 English manors.

The Lordship of Glamorgan was one of the most powerful and wealthy of the Welsh Marcher Lordships. The seat was Cardiff Castle. It was established by the conquest of Glamorgan from its native Welsh ruler, by the Anglo-Norman nobleman Robert FitzHamon, feudal baron of Gloucester, and his legendary followers the Twelve Knights of Glamorgan. The Anglo-Norman Lord of Glamorgan, like all Marcher lords, ruled his lands directly by his own law: thus he could, amongst other things, declare war, raise taxes, establish courts and markets and build castles as he wished, without reference to the Crown. These privileges were only lost under the Laws in Wales Acts 1535–1542. Though possessing many castles, the main seat of the Lordship was Cardiff Castle.