You have to pay close attention to the ranges of the your traps and cannons, because of course, they're going to be useless if they can't reach their targets. Of course, this also means that incredibly long-range weapons, like mortars, have an advantage nearer the base, so they can fire while the enemies are moving up. Place mortars behind your wall barrier, where enemies will have to stay fairly still to tear it down, and you're likely to do heavy damage.