At the Gate House to the Stanley Park Pitch and Putt, right around the corner from a very spiny Colletia hystrix (Colletia paradoxa? Serious spines way larger than the leaves) is another very spiny thing, that I figure must be Citrus trifoliata. I found the name from a posting somewhere calling it Poncirus trifoliata. I would guess this was deliberately planted (as opposed to its being the rootstock of another citrus), as it seems unlikely that a lemon or orange would have been planted here, and then there's the spiny plant theme going on. I see there was a BPotD posting on this species: Citrus trifoliata.
Perhaps it was planted to discourage access to the windows. I recently noticed the reassignment from Poncirus to Citrus as well.
Except for last year when I saw another Citrus trifoliata and didn't recognize it at all and posted it for ID, I have not thought about this plant, but it caught my eye last week. I think it deserves some photos here that show it off a little better.