Montgomery Castle had been built in the 1070s (not long after the Norman conquest) by Roger, Earl of Shrewsbury who named it after his home in Normandy and had became the property of the crown in 1102 when Robert of Montgomery rebelled against the king. It was in an important location just east of the ford of Rhyd Whyman over the river Severn.
Montgomery was held by Baldwin de Boulers, his son Stephen (slain by Llywelyn ap Madog of Powys in 1152) and grandsons Robert and Baldwin who succeeded him. The de Boulers lost the Lordship after they were overrun by the Welsh in 1207. It is presumably from the first or second Baldwin that the Welsh name of Montgomery, Trefaldwyn (a mutation from Tre Baldwin) arose.
The castle fell into decay although it was briefly reoccupied in 1223 when the area came back under English control and the new castle of Montgomery was built.
The original Montgomery castle of the de Boulers is on a site known as Hen Domen and is situated to the west of the modern Montgomery town (Powys, Wales).
What was once a timber castle is now a large earthen mound (c8 metres high and 40 metres in diameter) with the earthworks of the bailey. The site, which is now partially covered with trees, is the most extensively excavated timber castle in Britain.