Easiest way to dominate this game is: Get about 20 skill points, max damage and recruiting. Put mages near the start with archers backing them up/harvesting. Upgrade all mages with Wizard's Pal weapon. Once you have about 8 or so upgraded start using mana gained for casting RAWR spell. This dominates every map with ease. Got up to like +300% damage on all units before level ends.
LVL 18: Beat it using nothing but shadow Doomsday towers and canon towers upgraded only enough to get ice. Don't spread out your towers too much, make it so that the ice towers cover as many doomsdays as possible also make sure that enemies are always slowed with ice when in range of your doomsdays. GL.
Decent game over all. Areas to improve: Needs a cancel placement option, tower descriptions, return to menu/restart level option, display your base or whatevers hp.
You also need to adjust the strengths and abilities of the towers. There's no point to building anything except the $500 big damage tower since nothing else even comes close. Especially the $750 tower... it needs help. The freeze and poison type towers need to proc more often.