The mountain's name translates from the Gaelic language as Mountain of the Castle, a name which reflects the fortress like nature of its western flanks which fall steeply and rockily to the Auch Glen. The former spelling of Ben Achastle confirms that it is a name which goes well back into history. However the hills name may not just come from its physical appearance as W.J. Watson in his book Circular Forts of North Perthshire (1913) reveals that a circular fortification stood on one of the spurs of Beinn a' Chaisteil and the name may come from that. The fort has long since been obliterated by landslips but was the most westerly of over 20 forts which existed across northern Perthshire.