Old Castle Lachlan is a remote ruin on the picturesque shores of Loch Fyne. The ruins date back to the 15th century, replacing an earlier fortification on the same rock promontory. It was built by the clan Maclachlan, one of the oldest Scottish Highland clans and staunch Jacobites.

Following the death of the 17th clan chief at the Battle of Culloden in 1746, the castle was bombarded by government ships and left uninhabitable.