That peace was shattered when Civil War broke out in Britain. Although most of South Wales was Royalist, fortified Pembroke declared for Parliament. Pembroke was besieged by Lord Carberry but was saved by Parliamentary reinforcements arriving by sea at nearby Milford Haven.
The Roundheads then went on to take other castles in the area, including Tenby, Haverfordwest and Carew.In 1648, when the Civil War was all but over, Pembroke's leaders unexpectedly changed sides. Cromwell himself came to Pembroke and took the castle after a seven-week siege. Pembroke's three leaders were found guilty of treason and Cromwell ordered the castle to be destroyed.