The castle and earthworks comprise a linear development running north west-south east and lie in four separate areas of protection. At the north west end is a keep, located in the north west corner of the inner bailey. The inner bailey, in turn, lies in the north west corner of the outer bailey, which is now bisected by the modern road. Some consider the eastern part of the outer bailey to be a separate 'Eastern Bailey'. To the north of the outer bailey, on the east side of the modern road is a linear bank, which is considered to be associated with the castle or with the associated borough. To the south of the outer bailey is an enclosure with banks which was part of the medieval borough. Beyond this to the south east are the earthworks of a ribbon development of medieval occupation. Separate from this, and further to the south east is a complex of irregular earthworks representing agricultural enclosures and occupation areas. Longtown Castle was built by Walter de Lacy, a lord of the Welsh Marches in the late 12th century, to defend the English borderlands from Welsh raiders, and to protect the adjoining colony town of Ewyas Lacy, later known as Longtown. The town of Ewyas Lacy was established by Walter de Lacy at about the same time as the castle. The settlement was one of several new towns in the area, which were usually sited adjacent to a castle for defence. The smallholdings, or burgage plots, to the south east of the castle, and fronting onto the road were occupied by burgesses who paid an annual rent to Walter de Lacy and his successors. Further income was derived by fees paid by stall holders at a weekly market, and two annual fairs. The triangular market place has been largely built over, and St Peter's Chapel, which lay in this area, has become a house. The castle and town passed from the de Lacy family in about 1230. The town appears to have prospered for a time, and by 1310 the population is thought to have been more than 500 people. Following the Black Death in the mid-14th century, the town's population probably decreased. By 1540 the declining town was known as Longa Villa in Ewias Lacy, no doubt because of its linear settlement along the High Street, and from then on became known as Longtown. The keep is a stone built circular tower, thought to have replaced an earlier wooden one. It provided living accommodation in times of peace and a last line of defence in times of war. Although ruinous it can be seen that it had two storeys above a basement, and is constructed of shaley rubble with cut ashlar stone around the door and windows. Inside the keep are the remains of a spiral staircase near the entrance and a fireplace at first floor level. There is also a well inside the keep and two projecting garderobes or lavatories. The keep stands on a large earth mound, or motte, about 6m high, built for defensive purposes and to raise the height of the keep so that it provided a good lookout position. The inner bailey measures about 20m by 40m with a bank 2m high and 8m wide. There are remnants of some of its stone curtain walls intact. These wall fragments stand to about 3m or 4m high, and would have replaced a wooden palisade on top of the bank. The inner bailey contained accommodation for the castle's owner, his servants, soldiers and livestock. Linking the inner and outer bailey is a stone gateway with a bastion either side. The gateway has a double arch and is about 1.8m wide. The ground level has risen over the centuries to make the arch now seem very low, but originally a horse and rider could have passed easily through it. Originally there would also have been a portcullis to defend the gateway. The outer bailey, measuring about 100 sq m, would, in medieval times have contained several buildings mainly used for storage. Earthworks thought to be the remains of such structures can be seen in the eastern part of the outer bailey. The whole outer bailey is defended by a bank and ditch. The bank stands to about 2m to 2.5m high internally and twice that height externally, with the ditch about 6m wide. There is a modern entrance on the east side, but the original entrance is thought to have been on the south side, where there is a break in the defensive bank, and a high inturned bank. The entrance to the outer bailey would also have had a gateway. To the north of the outer bailey, on the eastern side of the road is a linear bank, on the same alignment as the bailey bank and ditch, about 2m high and 8m wide. To the south of the outer bailey is another large defensive enclosure, about 160m north-south by 140m east-west, bounded by a bank between 1m and 2m high on its west and south sides, although the eastern bank can no longer be seen. This area, part of the original medieval borough, has a lot of later building development which has modified some of the archaeological remains, but there are some areas of open space where archaeology will survive. Adjoining the south east end of this enclosure are further earthworks, about 0.5m to 1m high, in the form of six terraces or platforms, each measuring between 20m and 30m wide and about 60m long, fronting onto the modern road. These are crofts and tofts, the remains of medieval house plots, fronting onto the road, with their gardens behind. In the centre of this cluster is a later rectangular earthwork, measuring about 38m by 18m. To the south east of these, some 230m away, are more earthworks, about 1m high consisting of agricultural enclosures and house platforms. Some suggestion of a possible partial use of this land is indicated by the field name Pigeon House Field. To the east the land drops about 4m to level out near the stream. There have been a number of excavations and archaeological observations at the castle. Excavations in the early 1960s took place within the castle bailey. An excavation by Jarrett and Jones in 1966 suggested that there was no ditch within 7m of the earthwork to the north east of the castle. Further excavations took place within the inner bailey in 1972, and of the keep in 1978. A watching brief in 1995 found a stone structure built against the outside of one of the castle walls, and at the same time part of the surface of the motte was examined, but there was no evidence of a timber structure. Evidence of the tower construction trench was noted in two places with ashlar walls set on footings of pitched and vertically-set rubble. It is suggested that the great tower had partially subsided into the motte. Since the 1980s geophysical survey has been used to study several parts of the castle and borough, but these have often been inconclusive. (Scheduling Report)
The earthworks of Longtown Castle comprise a sub-square enclosure of approximately 1.21ha, defined by a single bank with an external ditch. Internally the ground level is higher due to the natural rising topography, although excavated evidence has shown that the change in level has been enhanced by an overburden of debris which has accumulated to a depth of 1m within the bailey (Ellis 1997, 71). The bank therefore rises as much as 4m high externally, but only about 2.5m internally. The ditch survives on the northern, southern and eastern sides where, although heavily silted in places, it remains up to 1m deep. On the northern side the former ditch now carries the road to Llanwonog. Here its outer edge has been partly filled in to accommodate houses and gardens which stand 1.3m above the road surface behind retaining walls. On the western side of the enclosure the ditch has been filled in to accommodate Jews Lane. We do not know whether the ditch originally carried water, although the western arm was under water at the time of the survey and two natural springs issue into the ditch. A field on the eastern side of the ditch was also named Cae Ffynnon meaning 'well field' on a map of c1800 (Jeffreys c1800).
There are six gaps in the rampart. In the eastern side, one lies close to the north-eastern corner of the enclosure, where a small footpath from the direction of Llanwonog and the Salem Baptist Chapel cuts across the rampart diagonally and leads towards the present village. To the south of this is a larger breach flanked by inturned ramparts and a corresponding causeway across the ditch. From its asymmetric position and form it seems unlikely that this is an original entrance to the enclosure, although it could still be of some age. It now has no obvious purpose, other than as a farmer's entrance, but it is flanked by two small building platforms and leads towards a complex of probable settlement earthworks. A further breach lies immediately east of the motte and was probably created when the motte ditch was cut. Two breaks in the rampart exist where the main village road runs through the enclosure breaching its northern and southern sides. Stonework on the western side of the road at the southern breach has been interpreted as part of a possible gateway (RCHM 1931, 183). Further stonework was seen protruding in two places in this area, one at the foot of the rampart and one close to its top. These appear to be the remains of a revetment wall. Close to the centre of the southern rampart, only 20m west of the break through which the road passes, is a further gap in the rampart. This carries a hollowed track-way which runs in a northerly direction into the centre of the enclosure. It is the only one of the breaks in the rampart which lies symmetrically with the enclosure, being situated centrally in its southern side. The fact that it also carries a hollowed track-way and is flanked by imposing ramparts on either side suggests that it may have been the original entrance.
The steep conical castle motte straddles the north-western corner of the enclosure. It is an imposing feature, standing approximately 11m high, with a circular stone keep on its summit. Its ditch has been filled in but traces of it can be seen in the inner bailey where a broad shallow depression, at most 0.3m deep, curves around the foot of the mound. Further traces can be seen to the west, where a further shallow ditch lies between Jews Lane and the mound and also where the enclosure rampart has been cut immediately to the south of the mound. As with the main enclosure ditch, the motte ditch carries a track on its northern side which has been partly filled in to accommodate houses and gardens behind a wall. This wall retains an external ground level of 1.7m.
The keep is a massive round structure constructed of local shale-like sandstone rubble with ashlar dressings. It has been the subject of several detailed studies (eg Morriss & Williams 1991; Remfry 1997, 34-35, 39). Its walls, 4m thick, survive two storeys high above an under-croft which penetrates its earthen mound. It has suffered partial collapse on its south- western side, where two large fragments of masonry lie on the slopes of the motte. The positions of the floor levels are marked by ledges in the interior. Three half round turrets are equally spaced around the outside face of the tower, one containing a fireplace, one a spiral staircase leading to the upper floors and one situated next to a projecting garderobe. The turrets do not show on the ground plan because the keep has a chamfered plinth. This is now robbed of its facing stone. The first floor windows have been enlarged at some point and decorated with fragments of re-used 12th-century mouldings. The 13th century, the date of the earliest fabric of St Peters Church, would seem a likely date for these modifications because episodes of church re-building may coincide with modifications to the seignurial site, perhaps using the same craftsmen (Creighton 2002, 110). The re-used masonry may have come from a 12th century chapel, perhaps situated within the castle. Chapels tended first to be in the keep and from the 13th century onwards at the side of the bailey and against an exterior wall (Pounds 1990, 240). Round keeps are rare and the example at Longtown is one of a group of 19 such keeps to have been recognised in the Brecon region (Shoesmith 1996, 170). (Smith 2003)