The Monnow Valley was an important route between Hereford and Monmouth in medieval times, due to its position as an area of relatively open land, which provided a break between the river cliffs of the Wye Valley to the east, and the hills around Abergavenny to the west. The Three Castles are usually grouped together by historians because for almost their entire history they were part of a block of territory under the control of a single lord.
All three sites have evidence for early Norman earthworks, possibly built by William fitz Osbern, who was made Earl of Hereford by William the Conqueror a few months after the Norman Conquest of England in 1066. From his castles at Monmouth and Chepstow, William was the first Norman lord to conquer central and eastern Monmouthshire, including the future sites for the Three Castles. The defences raised at this time would probably have been of earth and timber.
Fitz Osbern died in 1071, and his lands were forfeited to the crown after his son Roger de Breteuil was involved in a rebellion against King William in 1075. Later the king divided up this strategically important territory – the only time in their active history that the Three Castles were owned separately. They were reunited by King Stephen in the late 1130s as a response to Welsh rebellion in the southern March, and would remain a single lordship until the nineteenth century.
There is little evidence of building activity at any of the castles until the late twelfth century, when they were fortified by Ralph of Grosmont, a royal official who supervised building work for the king in Hereford. The castles were then completely overhauled by Hubert de Burgh, who was granted lordship of the Three Castles by King John in 1201. Control of the Three Castles was briefly granted to William de Braose in 1205, when Hubert was a prisoner of Philip Augustus, the king of France, but William quickly fell out of favour, and by 1207 John had forced him into ruin. Hubert de Burgh returned to power, and was appointed Justiciar in 1215.
From his time fighting in France Hubert had a knowledge of the latest in military architecture, and in the years after 1219 he was a prosperous lord who had great influence with King Henry III. He rebuilt Skenfrith Castle between 1219 and 1223 and Grosmont Castle between 1224 and 1226 in stone, adding domestic apartments to both castles, so that they could be used as lordly residences. He held the Three Castles until 1239, although they were briefly taken from him after he fell out of royal favour in 1232 (they were returned after his reconciliation to the king two years later).
After Hubert de Burgh, the Three Castles were held in royal hands, and in 1254 Henry III granted them to his eldest son, the future Edward I. In the 1260s the southern March was threatened by the Welsh Prince Llewelyn ap Gruffudd, who annexed the lordship of Brecon, and attacked nearby Abergavenny. Gilbert Talbot was appointed constable of the Three Castles, and ordered to garrison them 'at whatever cost'. Although Llewelyn's attack on Abergavenny failed, the Treaty of Montgomery in 1267 recognized his southern conquests, and he was considered a significant threat.
1267 saw the Three Castles being granted to Edward's younger brother Edmund, Earl of Lancaster. Although the Welsh threat was soon subdued with the death of Llywelyn in 1282, the Three Castles were used as residences and centres for local authority. The castles passed down through the earls of Lancaster until the death of Henry of Grosmont, Duke of Lancaster, whose daughter Blanche married John of Gaunt, son of Edward III. John of Gaunt was made Duke of Lancaster in 1364, and the Three Castles would remain part of the duchy of Lancaster until 1825. John and Blanche's son, Henry of Bolingbroke, deposed Richard II in 1399 and became King Henry IV, at which time the Three Castles also became royal possessions once more.
Although the Three Castles briefly saw action during the rebellion of Owain Glyndŵr in 1404-05, they never again played a major role in military affairs. Henry VI carried out repairs to White Castle and Skenfrith Castle in the mid fifteenth century, but by 1538 the castles were abandoned, and ruinous.
In 1825 the duchy of Lancaster sold the castles to the duke of Beaufort, whose estate divided them and sold each to different local landowners in 1902. White Castle was given to the State in 1922, followed by Grosmont in 1923. Skenfrith passed through several hands before being given to the National Trust. All three castles are now conserved and maintained by Cadw, and are open to the public.
The remains of the castle as it stands today date entirely from Hubert de Burgh's work, when he totally rebuilt the castle between 1219–23. Excavation has shown that the castle sits on an artificial gravel platform, up to twelve-feet thick. Evidence suggests that a defensive ditch would have surrounded the site, with timber walls. This early castle probably dates back to just after the Norman Conquest.
Ralph of Grosmont is recorded as having spent 43 pounds on Skenfrith Castle in the Pipe Roll of 1186–87. In the same excavation that discovered the early Norman defensive ditch, a twelfth-century stone wall was found, which suggests that Ralph was building in stone. A well carved decorative capital of red sandstone from the same period suggests a building of high quality, possibly a keep or hall. The location of the stonework, close to the early earthen defences of the castle, suggests that such a keep or hall would have stood alongside the perimeter of the castle, just as is the case of the hall at Grosmont castle, which was built in the same period.
Hubert de Burgh leveled these early defences, and no visible trace of them can be found. His new castle was built in the style of a concentric castle (which was quite cutting-edge for the time), albeit on a very modest scale. The castle consisted of a round keep with three floors, surrounded by a curtain wall with a round tower at each corner. Around the wall would have been a moat with a stone revetment, as seen at White Castle. The moat was filled with water from a connection to the River Monnow, which passes just to the eastern side of the castle. The entrance to the castle was in the northern wall – today it is simply a gap, but an engraving by the Buck brothers in 1732 shows the remains of a simple arch of stone in the center of the wall. Along the eastern wall a flight of steps leads down to a lower archway which probably served as a water gate, giving access to the moat. Next to the south-east corner tower is a blocked archway which may have been a postern gate to the rear of the castle.
The curtain walls have a sloping batter (the wall slopes down to be thicker at the base than along the top) for extra defence, and would have included a wall walk all the way around the inner edge. Support holes in the curtain wall, just below the level of the wall walk, were to support a timber hourding, or fighting gallery, which projected out from the wall and protected the defenders atop the wall. Each corner tower was built with a solid circular basement, presumably accessed by a wooden ladder from the upper levels. The towers would have been entered on the first floor via a wooden staircase from the outside. There were no windows, just arrowslits, suggesting that the towers were purely for defence, not residence.
Within the bailey there was a two-story hall block running along the inside of the western wall. The ground level was filled in with gravel in the thirteenth or fourteenth century, when the level of the castle's interior was raised in an effort to combat winter flooding. A later room was added along the northern wall, forming these buildings into an 'L' shaped block. Given this room's size and east-west orientation, this may have been the castle chapel. The upper floor was divided into three rooms, and the fine quality fireplaces and stonework suggests domestic apartments, and possibly a Great Hall. On the southern end of this block of buildings was a square tank which was the castle reservoir.
Across the bailey, along the eastern wall between the south-east tower and the water gate would have been the kitchens. The lightly built foundations suggest that the buildings were timber, built up against the curtain wall, with stone fireplaces, hearths, and ovens.
The main residence for the lord of the castle would have been in the round tower-keep which sits at the middle of the inner bailey. Entrance to the keep would have been, like the corner towers, through a doorway above the ground level, reached by a wooden stairway from the bailey. The bottom level is again a basement, while the upper two floors would have contained apartments. A turret projects from the western side of the keep, this would have held the spiral staircase that gave access to the upper levels. The well-appointed apartments included large windows, hooded fireplaces, and a private latrine. The keep was topped by a circular wooden hourding, similar to the one that surmounted the curtain wall.
Very little alteration has been made to the castle over the centuries. The level of the castle was raised, as was mentioned earlier, and at some point earth was piled around the bottom of the keep, giving it the impression of being set atop a mound. A door was also cut into the keep at ground level, bypassing the first floor entrance. Along the western wall, an external tower was added. This tower is solid to the level of the wall walk, and was probably added in the thirteenth century.
Skenfrith is located on the B4521, five miles north-west of Monmouth, and is an open site, which may be visited free of charge at any reasonable time of day. The moat has been filled in, and the castle is now surrounded by a grassy lawn in the centre of the small village of Skenfrith. Three of the four corner towers still stand, as does its curtain wall up to the level of the wall walk. The round keep is intact, and the foundations of the hall block along the western wall have been excavated as well.
Skenfrith Castle features in the 2010 Doctor Who episode "Amy's Choice" as a ruined castle in the fictional village of Leadworth.