I remember making the best defenses, and always sending attack in strategic waves. Where first would be the ground soldiers, for example if I was an orc, the first wave would be grunts and trolls. the second wave is reinforcement ground with healers, then heavy units, and lastly i would send in siege. I'd leave a fair amount of units at the base in case the attacks failed, so I have units to support the towers. It's been a while sense I've played though.