In 1632, King Charles I was forced to search for new sources of revenue as there was a depreciation of the coinage in the country. The castle and estate, which had been poorly maintained, was sold to Sir Thomas Myddleton of Chirk. A survey stated that the castle was ‘not worth quarrying for its stone as the original quarries were easier to work’ and the gatehouse had ‘a decayed roof of slate and including the wooden joists, doors and hinges it was valued at not more than 5 pounds’.
In 1642, however, the English Civil War caused the quick repair of the castle at the Crown’s expense and it was garrisoned against the Parliamentarians. Four years later in 1646, Parliamentary forces under the command of Major-General Mytton were given the task of reducing the castles of North Wales and their first target was Ruthin.
The castle withstood an eleven week siege during which the walls were massively attacked with artillery. The defenders surrendered only when the attacking forces announced their intention to mine the walls. In 1648 the castle was partially dismantled after an act of Parliament was passed to stop the re-occupation of fortresses by hostile forces. The walls were torn down and the timber and stone carted away for use elsewhere. Many houses in Ruthin have stones from the castle.