Sir Thomas' son, the second Sir Thomas, took up residence on his marriage in 1612 and as MP for Denbighshire from 1625, found himself on the Parliamentarian side in the Civil War. Royalist supporters seized the castle in 1643, and held it for three years. Sir Thomas' Parliamentary forces meanwhile enjoyed some successes, including the capture of Powis Castle, although he could not bring himself to attack Chirk.
The castle was eventually regained by bribery and Sir Thomas' son (Sir Thomas III) installed as governor. By 1651, however, the general had changed sides, and further payoffs were needed to dislodge the Parliamentarian garrison. Chirk was nevertheless besieged and taken by the Parliamentarians in 1659 as punishment for the Myddeltons' support of the Cheshire Rising. At the last moment it sustained the damage they had for so long sought to avoid. Most of the eastern side was demolished, and much of the rest burnt, leaving the family with a huge rebuilding task after the Restoration in 1660.
A new stone range was now added on the east, in conjunction with the reconstruction of the curtain wall and towers. The new towers, although externally similar to their predecessors, had much thinner walls, while the range included a drawing room and long gallery at first floor level, with an arcaded walkway facing the courtyard beneath it. The old state bedroom in the south-east tower was given a new entrance from the long gallery. Sir Thomas III predeceased his father, and his son Sir Thomas IV, who came of age in 1672, supervised the decoration of the newly built rooms, completed, possibly with the help of William Wynde, in 1678. Only the long gallery survives to show the original style of this work.
Within the east range, the main structure of the castle was complete, although minor alterations continued to be made. After an abortive episode in 1762-4, when a scheme for a Gothic interior was abandoned at an early stage, the north range was extensively refurbished in neo-classical style by Joseph Turner of Chester in the later 1760s and 1770s, the drawing room being completed by John Cooper of Beaumaris in about 1796. In the 1820s, however, gothic vaulting was added, and from 1845 the interior was almost totally reworked in the Gothic manner by A.W. Pugin, architect of the Houses of Parliament. Most of these alterations have been undone in recent years, with the exception of the Cromwell Hall, where a collection of Civil War arms is displayed. The castle remained in the hands of the Myddelton family, who still own and work much of the estate, until 1978. It is now in the care of the National Trust.
Offa's Dyke runs through the park. It can be seen from the air beneath the waters of the artificial lake, and is visible as a low bank as far as Home Farm, west of the castle. South of the castle it is better preserved, running to the west of the track, and out into the fields beyond, beside the footpath. The magnificent wrought iron gate-screen at the entrance to the park was made by Robert and John Davies of Bersham between 1712 and 1719. It originally stood a little way in front of the main castle gate, and was moved to its present position in 1770 during the landscaping of the park.
Info from castlewales.com