Good concept, bad execution.
While it's a nice twist on the "Defend your castle" genre, I don't think that in reality, it works well because you have that many units on the battlefield that the framerate is just too slow and therefore not worth playing.
Also of note is the fact that any other unit besides the archer is fairly unuseful, as they will undoubtedly get stopped by the arrows, which means that while it's nice to have such variety in the game, ultimately, it's useless.
Since this is version three of Invasion, it must mean that you're willing to listen to feedback you get and make a new version of it. I'd quite like to see version four and sort of the importance of each type of unit, along with solving the framerate problem, when there are 100+ units on the battlefield, along with some other features, such as the ability to "Unleash All" units with one click, for example.
I look forward to seeing V.4