Great topic. My 2 cents: No successful attack without special equipment should go straight through the walls. A builder repairing stone blocks faster than destruction is a good thing. To win a castle you should do either: 1. go above 2. go under 3. go through it with siege machines Contrary mentioned the above and under way. I will elaborate: 1) placing a ladder next the wall (not a skybridge) so that troops can go over the wall should be a totally valid and encouraged tactic. Defenders should try to destroy the ladder or use other equipment (oil? rocks?) to kill the attackers. 2) you can dig under the castle, unless it is built on bedrock. If so this way is impossible unless you have lots of bombs. Which you might develop over time with an armory (to be added) 3) Building a ram or another siege machine will be the way to go through a wall or steel doors. I think steel doors are a good idea. They should be invincible against swords and hammers but destroyed fairly quickly with a ram.