An Caisteal is a mainly grassy hill with some rocky outcrops on its higher slopes. The mountain is formed by four ridges which radiate from its summit, the north ridge goes over a series of undulating rocky knolls known as Twistin Hill to reach the subsidiary top of Sròn Gharbh (709 metres) before dropping down to the A82 road in Glen Falloch. This northern ridge is the most popular route of ascent and Twistin Hill has some curious features on it as it approaches the summit, there are a number of small caves close to the path and a deep cleft cuts into the ridge. The north west ridge also terminates in Glen Falloch after crossing the lesser top of Stob Glas (710 metres), this ridge can be used for ascent but is not as popular as the route over Twistin Hill.