Well done KeithG. Also described as the guardhouse to Fort Clarence. Opposite the old Girls Grammar School and adjacent to the Alms Houses. We lived almost opposite in Morden street. Used to play in there as a kid in the late 50s, early 60s as the wall was so easy to get over. Use the railings to get up on the ledge, lay across the plinth, spin yourself round, and down the other side.
Unfortunately a couple of kids got injured; I believe a fatality or two; and it was demolished in the mid 60s.
Surprisingly most of the view apart from the guardhouse is still the same.
Over to you!