Established by William Fritz Osbern, Earl of Hereford, soon after the Norman Invasion of 1066, the castle was a strategically located stronghold guarding the river crossings that linked the Forest of Dean, Celtic Gwent and Archenfield. Henry V was born here in 1387. During the Civil War it was held by both the Royalists and the Roundheads.
Only a fragment is left of this once important castle; the curtain wall, gatehouse and great round keep, which stood until the Civil War where the Great House now stands, have all completely vanished. All that is left is the ruined Great Tower and Hall. These stand on the edge of a precipitous slope down to the river Monnow, on the west side of what was the castle ward. This was roughly circular, surrounded on the west and north by the river and on the east and south by a wall and ditch, which is still partly apparent in the gardens behind Agincourt Square. Half-way along Castle Hill Road was the entrance, consisting of a bridge and strong gatehouse.
William fitz Osbern chose this strategic position, guarding crossings of the Wye and Monnow rivers, for one of his marcher castles sometime between 1067 and 1071, when he died. The Great Tower is similar in style to that at Chepstow, and was certainly built by about 1150. What can be seen are parts of its east and south sides. The west side fell in 1647, the north-west side remains but can not be seen from the town side, and a house lies over the rest. This was a fine early Norman rectangular two-storied building with the hall and main apartments on the main floor and a cellar or undercroft below. The east wall displays some Norman features: the small round-headed windows, the fragment of simple string course and the flat pilaster buttress in the south-east corner, one of a series which originally continued all along the wall. The entrance was at first-floor level on the south side. The castle was held uneventfully by Norman lords as the headquarters of an independent lordship until 1267, when it was granted with the Three Castles (White, Grosmont, and Skenfrith) to Henry III's son, Edmund Crouchback, when he became earl of Lancaster. He immediately built the large rectangular building to the south of the Great Tower, known as the hall. It was a single storey building containing one large room used for the holding of courts. It continued in use as such right up to the 17th century.
All the walls, except the north, stand almost to their full height. The entrance was the gap in the wall in the north-east corner, on the west side of which a moulded base of a door jamb is visible. The fireplace was in the middle of the north wall, and there were windows, now blocked, in the south wall. A later medieval window, also blocked, was inserted in the east wall. In the mid-14th century, during the lordship of Henry of Grosmont, 1st duke of Lancaster, the upper part of the Great Tower was transformed by the insertion of large decorated windows. The elaborate frame of one of these is visible in the east wall. The original entrance was replaced by a tall door, and the tower was re-roofed. At this stage, 12th-century corbels of carved heads were reset high up in the east hall, where they are still visible. This tower was almost certainly the birthplace of the future King Henry V in 1387. The end came for the castle in the Civil War, when it changed hands three times and was eventually slighted by the Parliamentarians. A local man's diary for 1647 records that on 30 March the townsmen and soldiers began pulling down the great round tower, which stood where Great Castle House now stands, and that on 22 December 'about 12 o'clock, the Tower in the castle of Monmouth fell down, upon its side, whilst we were at sermon'. The Great Castle House, built in 1673 by Henry Somerset, later the duke of Beaufort, was to replace Raglan as his family's residence in the country following the Civil War. In 1875 the house became the headquarters of the Royal Monmouthshire Royal Engineers (Militia), and as such it remains one of the few British castles still in military occupation. (Castle viewing from nearby parade ground only).
Great Castle House constructed from the original castle stone by the Marquis of Worcester in 1673. Its architecture had great impact on other buildings in the town. For many years it has been the home of The Royal Monmouthshire Royal Engineers.