A very fun game, it certainly has a different take on a tower defense game.
It feels more like an RPG strategy game than a tower defense game, given the fact that every level presents a unique challenge much like in your typical rpg, while in most tower defense games focus on very little difficulty progression and introduction to new gameplay elements/mechanics. This game shows us how a tower defense game can be very fun, intuitive, and tactical.
I also really like the upgrade system because it adds the ability to further customize how you play the game, in addition to giving the game replay value.
The unique attackers and spells as well as scrolls, and items that enemies drop give the player more feeling of interaction with the game world. The fact that tower placement is a decision that can cost you mana because you may need to cut down trees also adds a good bit of complexity to the game itself.
I often find myself planning which buildings I am going to seize first when a new round starts, which further deepens the gameplay. If this game was sold on steam I'd buy it in a heartbeat.