Slough Fort was initially armed with seven 7-inch rifled breech loaders (RBLs) positioned in the casemates. It was manned by three officers, one NCO and 75 other ranks. Between October 1889 and December 1891, a pair of wing batteries were constructed from concrete on either side of the fort, with a range finder position being added to the fort's roof. The new batteries accommodated two 9.2-inch and two 6-inch breech loaders on disappearing mountings, allowing them to recess below ground level once they had fired. These were much more powerful and longer ranged weapons than the old RBLs and were capable of engaging a battleship at a substantial distance. The old RBLs were removed.