Gun crews on the flanks of forts were particularly vulnerable to shells bursting overhead and behind the parapets. To afford them some protection guns were often mounted inside arched casemates, called Haxo casemates. These were named after the French General Haxo, who first proposed their use in such positions.
Fort Nelson has a two-gun Haxo casemate on each of its flanks. Inside both casemates of each Haxo was mounted a 7-inch Rifled Breech Loading gun capable of firing case shot to clear the approaches of the fort from attacking troops or common shell to dismount and destroy enemy guns and breeching batteries.
The embrasures of the Haxo casemates were protected by earth banks called merlons.
These prevented enemy shell fire from hitting the brickwork of the Haxo.
The West and East Haxos at Fort Nelson were of two different architectural styles. Not all of the Portsdown forts had Haxo casemates. Fort Nelson did because its flanks were considered to be vulnerable to an attacking force because of its position on the West end of the Portsdown Hill line.
The West Haxo casemate at Fort Nelson