Don't know if you're still looking, or still interested - but I was on the Island at the weekend and remembered your query about the tower in the Villa Marina grounds, and went and had a look.
It is very obvious as you walk into the Villa Marina gardens from the Promenade, standing at the top of the wooded slopes at the back, and at the top of a winding and, just now, very muddy path. From there it seems to be in a straight line with the war memorial and the tower of refuge.
It is made of the same rough chunks of stone as you can see in the back walls of many of the older houses and walls in Douglas. It is hexagonal in shape and castellated, as you remembered. Underneath the castellations the stone work is arched, but there are no openings apart from a doorway in one side, accessible from the bowling green. The inside measures about 5 feet across and it is about as high as a house. The walls are quite thick, about 2 feet I think. Inside there are no features, not even a roof. It seems to have attracted pigeon droppings and beer cans!
'Kevin' who was working in the Villa on Sunday morning told me it was originally a water tower for the old house, belonging to one of the Nobles, although some believe it was merely a folly.
You're right, it's not exactly genealogy, but someone might be interested ....