Denbigh Castle stands in a prominent position on a rocky hill above the town of Denbigh. After King Edward I's first major campaign (1276-77) to break the power of the Welsh prince, Llywelyn the Last, he agreed that a large area of land between the Conwy and Clwyd rivers should belong to Llywelyn's brother, Dafydd ap Gruffudd. Dafydd established his principle stronghold at Denbigh, and it was from here that he launched his own uprising against the English in 1282. Edward returned to Wales even more determined to crush the Welsh. Llywelyn was killed in an ambush and Dafydd was captured and executed at Shrewsbury. The stronghold at Denbigh was defeated and Henry de Lacy, Earl of Lincoln was given the new Lordship and tasked with building a new castle on the site.

King Edward and his master mason, James of St George, helped de Lacy plan the new fortification, which they then left de lacy to complete. Like the royal castles at Conwy and Caernarfon, Denbigh Castle was designed as an integral part of a walled town. The town walls were built first and then the castle was built within them. The most impressive feature of the castle is its unusual gatehouse, fronted by two polygonal towers with a third polygonal tower directly behind them forming a large triangular building. The remains of a statue thought to be of King Edward I can be seen above the entrance.

The incomplete castle was overrun during a Welsh revolt led by Madog ap Llywelyn in 1294. When the rebellion collapsed the castle was retaken by the English and construction worked carried on once more. However it seems likely that the castle wasn't complete when Henry de lacy died in 1311. Tradition says that Earl Henry abandoned the castle in 1308 after his son, Edward, fell to his death in the castle well. Ownership of the castle then passed through many hands, and construction work continued sporadically.

Owain Glyndwr failed to capture the castle when he began another Welsh uprising in 1400, and the Lancastrian Jasper Tudor was no more successful on two attempts he made in the 1460's. During the Civil War, the now decaying castle was repaired by Colonel William Salisbury and garrisoned for the King. In 1646 the castle endured a six month siege before finally being forced to surrender. For the remainder of the war the castle was used as a prison for captured royalists, but after the restoration of King Charles II the castle once more fell into decay and ruin.