Limited fireproofing was indeed achieved on these machines. It was a fabric or skin composite, with an added mixture of vinegar, seaweed, mustard and water. I don't remember the rest. The fireproofing was very efficient mind you, but it only covers the front and sides of siege towers, and it's only a shell. The trick was to send incendiary projectiles that would -penetrate- the hide and make the inside catch fire. There was plenty of oxygen to feed such a fire.