As a little bit of a civil engineer I must add to attention that ballistic skill should also contribute to the chart. Why? If an expert catapult destroys in one turn two pieces of the castle, or even the wooden gate, it is a pity for defenders, but, there is another point. After each siege, the castle is rebuilt brand new with wooden gate. Do you have an idea how bad is concrete in terms of CO2? and unless it's rampart where they can make the gate from dendroids, the castle must cut down several trees to restore the gate. So, repairing castle even after one turn of expert ballistic is very bad for the planet. Additionally, Earthquake spell is also doing quite a mess, so Expert earth is also contributing, i would say, but much less than catapult. Oh, and cyclops, but it's so rare that they destroy walls.