Why boiling oil when defending castles? Would not boiling water work as well?
I would say that the oil has the advantage of being flammable, so a well-placed burning arrow, or just dropping a torch, could light up the unlucky peons that were doused with it.
Water might instead extinguish any defensive fire.
At the very least I would guess oil holds heat longer than water, which when spread thin cools quickly. More embodied energy in the oil.
To add to Avocado's points, the oil is also going to slick the walls and the ground near them, making them harder to scale or assault.
Historically, boiling water was what was generally used. Oil sticks to skin, making it more effective. But water is cheaper and easier to come by.
Depending on the oil, you can get it up to at least 400 degrees. Water tops out at 212.
This thread is pure awesome.
Thik