Old Inverlochy Castle is a ruined late 13th century stronghold strategically situated at the southern end of the Great Glen, on the banks of the River Lochy, near Fort William. It is one of the few Scottish castles from the 1200s to survive unaltered. It was originally protected by thick curtain walls with round towers at each corner, defended by the river on one side and a water-filled moat on the other three. 

It is the site of two major battles. In 1431, the Earl of Mar and Caithness headed the royalist cause against Highland clansmen, but was soundly defeated. In 1645, the royalist Earl of Montrose routed the roundhead forces of the Campbell Chief Duke of Argyll at the second Battle of Inverlochy.

In 1654 the castle was abandoned in favour of a large timber fort built nearby by Oliver Cromwell. This was later replaced by a stone fort named Fort William.