The routine that created the "No touch" zones and rectangular rooms is again shared for the creation of fire tiles. A rectangle of random size and random width is created at a random position. This time, the routine looks for wall cells inside that rectangle and replaces them with fire cells. Easy as that.
I was surprised when I saw this, but it makes so much sense. It's a very easy and quick way to place fire without blocking an existing path and simultaneously preventing the player from going to places not allowed to. They're like see-through walls. Brilliant!
