Interesting game, quite amusing and certainly made me spend more hours than I expected, which is exactly what I like about TD games, and games in general.
As it is, I find the game quite enjoyable. The difficulty system is excellent, something I've seen on a couple other games, and I did enjoy it there as well. Not locking XP to higher difficulty settings is also a good move that caters for casual and hardcore player alike.
Perhaps a sequel could be done, as I certainly would like to see a sequel, maybe with hero units (unique units) with a longer spawn time but with triggerable skills (magic/melee, depending on the hero).
Congrats on your work and I do hope you continue producing more and better, as this is a clear indication of your talent.