Longtown Castle stands on the Welsh border at the base of the Black Mountains. It is a fairly standard motte-and-bailey earthwork, built by William de Lacy in the 1180's on the site of a square enclosure that may have been a Roman fort. In the early 13th century another Walter de lacy, Sheriff of Herefordshire, rebuilt the defences in stone. He built a curtain wall around the bailey, which he divided into two halves with a cross-wall, and he added a circular keep on the motte. The best preserved part of the curtain walls is the cross-wall in which there are the remains of a gateway flanked by two half-round turrets. The circular keep, uncommon in England but relatively popular in Wales, was two-storeys high. It was built on a sloping plinth and was strengthened by three semi-circular buttresses. The buttress that contained the spiral staircase has collapsed leaving a gash in the side of the tower.

The castle was abandoned in the 14th century, although it was temporarily refortified in the early 15th century during the Owain Glyndwr rebellion.