Day 093/20. Castle Waterboard ?

A walk today to the top of our local hill, well, we live almost at the top, so I guess its across to the top.....
At the top there is an underground reservoir providing fresh water to the neighbourhood with water pumped up from the treatment plan on the River Itchen at the bottom of the hill.
This castle like structure is, I guess, the entrance to a spiral staircase leading down into the reservoir itself. It looks like it has been a while since anyone has been in there though, as the locks on the door were rusted over.
Still, an interesting little building.