The market town of Abergavenny has a strategically important location along the River Usk, near where it emerges from a valley between the Black Mountains to the north and other hills in the south, continuing across the more open country of the Welsh Marches to the east. In ancient times the place was a centre for iron extraction, and was occupied by the Romans, who built a fort here, but it was after the Norman invasion that the settlement grew significantly, accompanied by construction of a castle, begun around 1087 under Hamelin de Ballon.
Like most early Norman castles, the fortification initially consisted of a simple motte and bailey, centred on a wooden keep. This was soon replaced by a stone structure, and the place was expanded and rebuilt several times over the next few centuries, remaining essentially intact up until the English Civil War, after which it was deliberately destroyed to prevent any role in future conflicts, and most of the masonry was removed for use elsewhere.
A hunting lodge was built on top of the motte in 1819, and is now used as a museum, while below are the fragmentary remains of the exterior defences, the largest section being two towers, a gatehouse and a tall section of the curtain wall, to the west; much lower walls enclose the rest of the perimeter, and inside, the majority of the enclosure is occupied by a lawn, partly lined by trees. The surviving components, although only a small part of the original, are still substantial and impressive, and the castle has quite a scenic setting, within sight of some of the easternmost hills of Brecon Beacons National Park, and shielded by the trees from the town's modern development. Abergavenny Castle is owned by the local council, and is free to enter.
The most significant event in the castle's first century was a massacre that took place during the Christmas of 1175, at which time the fort was owned by William de Braose, 3rd Lord of Bramber. In retaliation for the killing of his father-in-law (Henry FitzMiles) several years earlier, the supposed culprit, Seisyll ap Dyfnwal, was invited to a banquet, ostensibly as reconciliation, but was summarily executed, together with his son and other companions. Retribution for this quickly followed, involving more killings, and partial destruction of the castle. Today, all the visible remains are from the replacement structure, building of which started at the end of the 12th century, using the local Old Red Sandstone, quarried from nearby hills - a new stone keep on top of the motte, and a high wall all around the outside of the bailey, strengthened by five towers at intervals. Further improvements were carried out during the 13th and 14th centuries, principally addition of two larger towers along the west side, both four stories tall, plus a great hall, a chapel and a higher curtain wall. The final component was a two-storey gatehouse, also on the west side, at the start of the 15th century, but use of the castle declined soon afterwards and it was unoccupied prior to the Civil War. The main subsequent development was installation of the hunting lodge, by Henry Neville, Earl of Abergavenny; also in the 19th century the grounds were cleared and part was transformed to a formal garden, though the ruined sections survive without any modern interference.
Abergavenny Castle is situated just south of the city centre, on a low hill about 50 feet above the River Usk and 1,000 feet distant, close to the confluence with the smaller Gavenny River. The large Castle Street car park is close by, from where visitors walk along a short lane and enter the castle through the ruins of the 15th century gatehouse. The main ruined section adjoins this to the south - a high wall extends a hundred feet to the remains of the twin, four story, residential towers (one circular in cross-section, the other polygonal), and a shorter wall continues on the far side. Other parts of the curtain wall are either completely missing, just a few inches high, or (on the east side) part of the boundary with private properties beyond; along this section are minor remains of two smaller towers. The western half of the site can be viewed from both inside and outside the perimeter walls, while the central section is occupied by the museum, within the former hunting lodge, on top of the original motte, together a few other modern buildings, and the garden. Other traces of the medieval structure include short stretches of internal walls, some from the great hall, two staircases, and the entrance to a cellar, or dungeon, this fenced off and closed to entry.