A Vision of Britain through Time provides the following description of Thirlwall from John Marius Wilson's Imperial Gazetteer of England and Wales of 1870-72:
Thirlwall was a township in the ancient parish of Haltwhistle and became a separate civil parish in 1866. Thirlwall remains a separate civil parish.
Thirlwall Castle is a 12th-century castle in Northumberland, England, on the bank of the River Tipalt close to the village of Greenhead and approximately 20 miles (32 km) west of Hexham. It was built in the 12th century, and later strengthened using stones from nearby Hadrian's Wall, but began to fall into disrepair in the 17th century. The site is protected by Grade I listed building and Scheduled Ancient Monument status.
The home of the Thirlwall family, it was fortified in about 1330 by John Thirlwall. In a survey of 1542 it was reported as in the ownership of Robert Thirlwall and in a 'measurable good' state of repair.
Sir Percival Thirlwall of Thirlwall Castle was killed at the Battle of Bosworth whilst fighting in the Yorkist cause on 22 August 1485. He was Richard’s standard-bearer in the final charge at Bosworth.
Eleanor Thirwall, the last of the Thirlwall family line, abandoned the castle as a residence and the estate passed to the Swinburne family by her 1738 marriage to Matthew Swinburne of Capheaton Hall. Swinburne sold the estate to the Earl of Carlisle for £4000 in 1748.
Thereafter the castle fell into decay. In 1832 and again in 1982 there were serious collapses of masonry.
In 1999 the Northumberland National Park Authority took over the management of the castle, protecting it from further dereliction.
(Various references for more information on Wikipedia.