The ancient remains of its castle are some of the oldest in Wales - commenced in 1277 by order of Edward 1 - years before the great fortresses of Conwy and Caernarfon.

Llywelyn ap Gruffydd's brother Dafydd started a castle at Ruthin but forfeited it when he rebelled with his brother. Edward I's queen, Eleanor, was in residence in 1281 so it must have been habitable by then.

Reginald de Grey, Justiciar of Chester and a marcher lord, was entrusted with the defence of Ruthin in 1277 and he completed the castle in 1284.

De Grey was given the Cantref of Deffrencloyt for services to the king, and his family held sway for the next 226 years when Dyffryn Clwyd was, in effect, an English Hundred. During the reign of Henry IV, the 3rd Baron de Grey sought the political downfall of Owain Glyndwr, who had been in favour at the king's court. As a result Glyndwr's lands were confiscated and he took revenge by attacking the town in September 1400. Only the castle and a few buildings were left standing.

Owain Glyndwr went on to become the most successful of many Welshmen who fought for and gained an independent Wales. In 1646 the castle survived an eleven week siege during the Civil War but was demolished in 1648.