Complex sentence: Two small fortifications on either side controlled access to Gibraltar, which could only be reached via a narrow causeway between the Inundation and the sea, which was enfiladed by batteries mounted on the lower slopes of the Rock.
Split into simpler sentences: Two small fortifications on either side controlled access to Gibraltar. The only road to and from the town ran along a narrow causeway between the Inundation and the sea which was enfiladed by batteries mounted on the lower slopes of the Rock.