In Welsh Denbigh Castle is known as Castell Dinbych.
The original Welsh castle on the site of the present Denbigh Castle had belonged to Llywelyn the Great. There are records of an Abbot from England visited Llywelyn the Great at his new castle in Denbigh in 1230.
Work on the castle that stands today was begun under Henry de Lacy, 3rd Earl of Lincoln on territory given to him by Edward I after the defeat of the last Welsh prince, Dafydd ap Gruffudd in 1282. The Welsh castle was torn down and work began on a new English fortress. At the same time, De Lacy was also granted a Royal Charter to create a new English borough and town. A planned town (bastide) was laid out at the as the castle as part of the attempt by Edward I to pacify the Welsh.
Denbigh Castle was built in two phases, based on designs attributed to Master James of St George. In the first period, commencing in the same year (1282), parts of the outer ward were constructed. These outer defences included the southern and western walls and the eastern towers. Later work on the inner ward began including parts of the curtain wall and the castle's main gatehouse. The borough's new town walls were also begun during this period.
In the 1290s, Edward I issued a second Royal Charter as the market town of Denbigh had rapidly expanded beyond the town walls and its borough boundaries.
In 1294, the incomplete castle was besieged and captured by Welsh forces during the revolt of Madog ap Llywelyn. An English force under de Lacy was defeated trying to retake the castle. However the revolt collapsed and Denbigh was returned to de Lacy a year later. Building work then resumed. Henry de Lacy substantially revised the plans in the second phase of building work. This time the inner ward's curtain wall were refortified with thicker and higher walls, complete by 1305.
By 1305 there were 183 settlers living outside the town walls and only 52 inside the town's defences. The castle and its precincts were being superseded by the area outside the walls which had developed into the town's market centre. A Carmelite Friary was also established in the town just outside the town walls.
The main gatehouse was heavily buttressed with a three octagonal towers and a drawbridge. Of the three towers faced outwards and a third interior tower, (the Badnes Tower), inside the main gateway. This three-towered triangular gatehouse created a heavily defended passageway that had murder-holes, portcullises in series, two wooden doors, and enfilading arrowslits. One gatehouse tower contained the porter's lodgings while the other served as the prison.
During the same period, work was also completed the Great hall and the eastern domestic ranges including the kitchen tower, the pantry and the postern gate. The town walls were finished, the eastern section was defended by several large D-shaped towers, such as the Countess Tower and Goblin Tower.
Work continued well into the 14th century. The fortress and borough removed all traces of the Welsh fortifications. De Lacy died before his new castle was completed.
In 1400, the forces of Owain Glyndwr attacked Denbigh. The town was badly damaged but the castle resisted a siege.
During the Wars of the Roses, Jasper Tudor, the Lancastrian Earl of Pembroke, tried twice and failed to take the castle in the 1460s.
In the 16th century Robert Dudley, Earl of Leicester, held Denbigh Castle and its Lordship between 1563 until his death in 1588.
During the English Civil War, the castle was garrisoned for King Charles I of England, who stayed there briefly in September 1645. The following year, the castle endured a six-month siege before surrendering to Parliamentarian forces. The castle was then slighted to prevent its further use. For the remainder of the war part of the castle was used as a prison for captured royalists.
With the restoration of Charles II in 1660, the castle was abandoned and allowed to fall into decay.