Gunpowder Trails & Gunpowder Stashes
Using gunpowder produced by your gunpowder workshops, you can  place chained gunpowder trails and gunpowder stashes around your  castle.
Archers on walls, towers and gatehouses can launch fire arrows,  setting them alight and causing a chain reaction along the adjoining  trails and stashes. Gunpowder trails inflict fire damage to any troops,  enemy or allied, standing nearby and gunpowder stashes inflict  massive damage to anyone standing near them when they explode. 

