Cool game. The idea is solid and just needs some tweaking to make a very good game :).
Good job on getting your art assets done and adding music :). The map changing colour each time is also a nice touch. The title screen to play transition is really cool.
I kinda like the difficulty. Though, I did play some Starcraft Broodwar lol
A very easy level 1 would be nice just for letting brand new players figure out the rules/controls. -- Just 2 enemy spawners, spawning half a fast, something like that.
The current group movement is pretty good :) , but I wish they didn't get stuck when walking straight.
With some more development this could be a very fun single player RTS.