How does boiling oil work?
To place boiling oil in your castle you need to build a smelter, this requires iron to build and needs to be researched (Military, Defences). Once the smelter is complete you will be able to place five oil pots on the castle walls. Each pot requires pitch to make it. If you want more pots, you need to make another smelter. Smelter’s work by themselves and do not require people to operate them.