The mountain reaches a height of 995 metres (3,264 feet) and is usually climbed together with the neighbouring Munro of Beinn a' Chroin,[2] to which it is connected by a high col to the south east.

The mountain's name translates from Gaelic as "the castle" and is believed to refer to the prominent castellated rocks at the summit which serve as an easy way of identifying it from a distance.

The south east ridge links to Beinn a' Chroin across the Bealach Buidhe (805 metres) and the headwaters of the River Falloch.

An Caisteal is usually ascended from Glen Falloch — there is a large lay by for parking on the A82 at grid reference NN369239 although it is also possible to start from Derrydaroch farm 2.5 kilometres to the south west.

[5] The route goes under the railway by a sheep creep and follows a track by the River Falloch upstream for about a kilometre before striking SW to climb Sròn Gharbh direct across pathless grass.