First built by Walter, Sheriff of Gloucester in the early 1100s, the castle was captured and destroyed on several occasions before being rebuilt in stone during the 1190s.

The castle is in the county town of Carmarthen located 20 metres (66 ft) above sea level on a high terrace overlooking the tidal River Towy.

[3] While it is described today as "arguably the biggest disappointment among the plethora of medieval ruins in Wales"[1] it has, in fact, dominated the layout and orientation of the town with its streets and property boundaries coming out from the site.

The castle, originally a timber motte and bailey structure, is believed to have been rebuilt in stone in the later years of the 1100s.

[9] It was not too long before local Welsh prince Gruffydd ap Rhys (who had had his lands taken from him by the Normans) attacked Carmarthen Castle.

Around 1116 he successfully captured it in a night attack, despite the Normans arranging for it to be defended in a two weekly rotation by local chieftains.

[1] Opposing Yorkist troops led by William Herbert[11] captured the castle in August 1456, imprisoning Edmund Tudor, who died a prisoner there on 1 November.

[6] With no further use as a prison, Carmarthenshire Council bought the Old Gaol in 1925, intending to build the new County Hall and a museum on the site.