This is a two gun battery sunk into the base of structure.  It can be reached by a set of stairs that run down the back of the demi-bastion.  It would appear that these two gun positions were originally open to the rear, to allow ventilation.  There is now concrete covering over the rear of the casemates which was installed during WW2.

These two gun positions were originally known as King’s Lines Place of Arms Casemates.  In 1859 there were 7 x 24 lbs guns int his position and we must therefore assume that the two casemates were built at some date after this.  By 1886 it would appear that the two casemates have been built and we have 2 x 32 lbs smooth bore guns in this positon.