Alright, this was a pain in the ***.
Written instructions to the last 11 tiles (since the screenshot isn't available any longer, and the map isn't extremely clear):
In the middle room (where Snafaru's tank is), go to the upper portal. It will take you to the room on the right. Go downwards to the other portal. It will take to the room on the left. Go through SAME portal. You're now in the upper room, where the last 11 tiles are, and where the last valve to set Snefaru free is located.