William FitzOsbern, the 1st Earl of Hereford, established the castle between 1066 and 1069, and the infobox gives 1067. It began as an earth and timber ringwork fortress, a typical Welsh Marches border castle. It was in the Domesday Book of 1086, and stonework was added before 1150.
Edmund Crouchback built the Hall after 1267, and the Great Tower received decorated upper windows in the early 14th century. Slighting began on March 30, 1647, and the round tower collapsed on December 22 that year. Henry Somerset, 1st Duke of Beaufort, built Great Castle House on its site in 1673.
The British Army owns it, and the Royal Monmouthshire Royal Engineers have used it as a headquarters since 1875. The article says it is one of few British castles in continuous military occupancy. It is Grade I listed, and a regimental museum is on site.

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