However, military uses for castles in Wales were not completely over. When the Civil War broke out between the King and Parliament in 1642, a number of Welsh castles were refortified to become bases for troops supporting King Charles I. New weapons – guns and cannons – were now in use, and walls were strengthened to withstand gunfire. Platforms were built outside the walls to mount cannons.
The Parliamentarian army besieged the castles at Conwy, Caernarfon and Harlech. Fighting was even harder in south-east Wales, where the earl of Worcester defended Raglan Castle and his son Edward held Chepstow Castle for the King. During the summer of 1646 a Parliamentarian army of 3,500 men led by Sir Thomas Fairfax besieged Raglan Castle and, when they finally took the castle, it was a serious defeat for the King. The castle was deliberately damaged to prevent the Royalists using it again.