Following the outbreak of the English Civil War in 1642, the castle was held by forces loyal to Charles I, holding out until 1646 when it surrendered to the Parliamentary armies. In the aftermath the castle was partially slighted by Parliament to prevent it being used in any further revolt, and was finally completely ruined in 1665 when its remaining iron and lead was stripped and sold off. Conwy Castle became an attractive destination for painters in the late 18th and early 19th centuries. Visitor numbers grew and initial restoration work was carried out in the second half of the 19th century. In the 21st century the ruined castle is managed by Cadw as a tourist attraction.
UNESCO considers Conwy to be one of "the finest examples of late 13th century and early 14th century military architecture in Europe", and it is classed as a World Heritage site. The rectangular castle is built from local and imported stone and occupies a coastal ridge, originally overlooking an important crossing point over the River Conwy. Divided into an Inner and an Outer Ward, it is defended by eight large towers and two barbicans, with a postern gate leading down to the river, allowing the castle to be resupplied from the sea. It retains the earliest surviving stone machicolations in Britain and what historian Jeremy Ashbee has described as the "best preserved suite of medieval private royal chambers in England and Wales". In keeping with other Edwardian castles in North Wales, the architecture of Conwy has close links to that found in the kingdom of Savoy during the same period, an influence probably derived from the Savoy origins of the main architect, James of Saint George.
Before the English built the town of Conwy, Aberconwy Abbey, the site was occupied by a Cistercian monastery favoured by the Welsh princes. The location also controlled an important crossing point over the River Conwy between the coastal and inland areas of North Wales, that Deganwy Castle for many years had defended. The kings of England and the Welsh princes had vied for control of the region since the 1070s and the conflict had resumed during the 13th century, leading to Edward I intervening in North Wales for the second time during his reign in 1282.
Edward invaded with a huge army, pushing north from Carmarthen and westwards from Montgomery and Chester. Edward captured Aberconwy in March 1283 and decided that the location would form the centre of a new county: he would relocate the abbey eight miles up the Conwy valley to a new site at Maenan, establishing Maenan Abbey, and build a new English castle and walled town on the monastery's former site. The ruined castle of Deganwy was abandoned and never rebuilt. Edward's plan was a colonial enterprise and placing the new town and walls on top of such a high-status native Welsh site was in part a symbolic act to demonstrate English power.
Work began on cutting the ditch around Conwy Castle within days of Edward's decision. The work was controlled by Sir John Bonvillars and overseen by master mason James of St. George, and the first phase of work between 1283 and 1284 focused on creating the exterior curtain walls and towers. In the second phase, from 1284 and 1286, the interior buildings were erected, while work began on the walls for the neighbouring town. By 1287, the castle was complete. The builders recruited huge numbers of labourers from across England for the task. At each summer building season the labourers massed at Chester and then walked into Wales. Edward's accountants did not separate the costs of the town walls from that of the castle, but the total cost of the two projects came to around £15,000, a huge sum for the period.