Great Game
This game is a whole lotta fun for tower defense fans. I love the variety and challenge. Here's what I think you need to improve on:
-Items should have an auto-sort feature
-You have the beginning of a rock-paper-scissors game between melee, ranged, and magic attacks. Unfortunately, the only "losing" one of these is melee, since they can't hit flying units. This balance just doesn't make any sense. Why not make some enemies with stone skin that can't be hit by arrows and another with a shield that stops magic attacks? This forces a wider variety of units and makes the melee units much more use-able.
-I absolutely love your unit leveling system. Unfortunately, I almost never had enough experience to level up any single unit as strong as it could be (I could usually level up the strongest units 5-7 times). It takes away some of the satisfaction of building a unit up. I'd prefer tougher baddies with higher experience payouts.
-Your hero and leader level up system is great too, but what about a way to redistribute skill points? One of the leader's skills just makes him level up faster. Why would I want to do that if I permanently lose the stat point? It's a useless skill! So work on this.
-The game becomes very tedious as it goes on. I think much of this has to do with my second bullet point, but a mini-game or some other sort of gameplay variety would be nice to help support such a large game.
I hope there's a sequel!