I'd like to see a bunker added as a new defensive building. You load it with your own ground troops. Upgrading the building lets you put more troops inside. The troops don't leave the bunker under an attack. When the bunker is destroyed all the units inside die with it. Let's say you can only have one type of troop inside the building. Total dmg per second is the sum of the individual units dps. Same goes for range, so please don't put barbarians in there.
