If you walk over the slopes of Mynydd Bach Trecastle, you can still see the route of a Roman road, and the remains of two Roman marching camps, as well as stone circles, the sites of hafods (summer dwellings) of the Iron Age, a small Norman motte, and evidence of later quarrying activities.
In about 1100, the Normans constructed a motte and bailey to protect Brecon from attack from the west. Known as 'una villa nostra de Lliwel', its local name was Trecastell - the Town of the Castle. However, uprisings continued and as late as 1295 King Edward I spent three days in Trecastle quelling a revolt.
From medieval times the land to the west of the Login brook was known as Tre- Esgob, or Bishopstown. Owned by the Bishop of St Davids and used as a 'staging post' for traffic to and from St Davids, it was doubtless used by Gerald of Wales on his 12 th century journeys. It is likely that the original route through the village ran through Bishopstown, and up to the Roman road, the first dwellings being built alongside the Login brook.
In the 19th century Trecastle had 8 annual fairs, its own gasworks, two schools, a corn mill, two smithies, 16 shops and numerous pubs. Between 1830 and 1914 Trecastle and Llywel are said to have had two of the most important woolen mills in Breckonshire. However, the village suffered a serious reversal in its fortunes when the railway arrived in Sennybridge, which then took Trecastle's place as the trading centre for the area.