Only the best TD game I've ever played. This should be the model for future TD games. I liked the idea of being able to build a castle, but eventually actually just made stand alone 2x2 towers with an open path to my garrison, that they never made it to. It also prevented the enemy from posting just out of range and shooting at my castle wall.
I'd like to see a longer storied mode, where the first battle was at the border and the last battle was defending your capital. The strength of the final army against you influenced by how well, or how poorly you defending the outlying keeps.
As for balance, yeah, units do become over powered soon. Necessary for the marathon round, but these unit bonuses could be adjusted to provide multiple difficulty levels.
Other than that, the development of some different defensive artillery and siege equipment.
And finally a random map generator for endless, every changing fun.
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