Unlike the other 3 VP structure, which simply looks as if it has sloped sides, this one actually makes good use of the third vanishing point.
This VP- more than the others- requires care in its use. In part, because you can see the underside of so much here (like the ledges and the top sills of the windows), it looks as if you are looking up at a very large thing. If it had been on the ground and you were a mere 100 feet away, you'd hardly use a third vanishing point at all for it.
The background shapes also illustrate something interesting here...
The Floating Tower looks as if it is nearing, and floating above wherever you happen to be. This is why using the Third Vanishing Point- the one shown- was appropriate.
But, since the background shapes are NOT so close, and do not appear to be floating at all, to have used that same vanishing point would not have worked too well.
What I did was to sort of slope them slightly, much less so than the Tower, but what I could have done- and had I thought of it before, would have!- was to have used a FOURTH Vanishing Point, this one well above the Third.
Thus, the background shapes would have sloped slightly, but still in an organized way.
The overall effect would have been to not only show that the Floating Tower was nearer and hovering well above you, but to have focused attention on the Tower. This would not have been the case (as much) if all had used the Third Vanishing Point.