There is one more school to attend, though, so please bear with me. You've mastered the art of laying out your dungeon, mustering your forces, and burning mana in the most delightfully evil manner possible. But still, one thing is missing. The best way to get a feel for doors and traps is to experiment. As I've said before, My Pet Dungeon is a great place for this, um, research.
Many traps work very well in combination with other traps and doors.
Experiment, so that you can create some combinations and layouts that work well for you. Try putting a freeze or lightning tarp in a niche in the wall with a sentry trap ahead. As the creature rushes the cannon, it'll get zapped by the other trap. Generally, traps work better in confined spaces like corridors. If you put traps in a large room, foes might just run past.