This Retrenchment Wall would appear to have been an early defence wall, improved during the later half of the 19th Century.
There is a set of steps running up from the Princess Lines to just near the Lower Union Gallery, the guns of which covered the dry moat and glacis of the Retrenchment. This was a stop line in the event of any attacking forces gain a foothold at the northern end of the Lines. The underground King’s Gallery has loop holed walls across the tunnels at roughly the same position.
There is a 3ins Mortar position, built during WW2, in the bottom of the dry moat.