|Photo by Warofdreams [CC-BY-SA-3.0], via Wikimedia Commons|
Anyway, its page on Wikimedia Commons labels it as, "Queen's Tower Lodge," so I'll assume that's what it is.
The rest of the Queen's Tower site is nowadays a gated apartment complex but what function does this particular building currently serve? I don't know. Occasionally, when you go past, you can spot a bare, lit bulb through one of the windows, suggesting some kind of activity within. But as to what goes on in there, we passers-by can only guess.