Hen Domen Welsh, meaning "old mound", is the site of a medieval timber motte-and-bailey castle in Powys, Wales.

When the castle was rebuilt in stone (1223–1234), it was decided to rebuild it on a rocky promontory a mile to the southeast—the location of the current town of Montgomery, Powys.

The castle was built on a low hill, only 0.7 km (0.4 miles) east of the Ford of Rhydwhymanan (an important shallow crossing of the River Severn), and offered good views of the Severn Valley to the north and west, and of the Vale of Montgomery to the east.

[7] Excavations from the 1960s until the 1990s have revealed traces of a series of grand and complex buildings including a towering timber building on the motte, connected by a bridge to the bailey with a large hall, 5 metres (16 ft) by 6 metres (20 ft).

[2][3] Hen Domen was built sometime soon after 1071 when Roger de Montgomery, the first Earl of Shrewsbury, was given extensive areas of Shropshire by William the Conqueror.