Excellent work on this!
First and foremost, I have to applaud the programming for the most competent AI system I've seen in quite a while! (AAA titles included) While the game will obviously have its issues, they are very few and scarcely visible if you're not looking for them.
Before I extol too much, I'll air the grievances:
First off, difficulty curve. I can most definitely appreciate a difficulty thaat doesn't let you just soam one unit to zerg unto victory, but you may want to take a closer look at the shifting between levels. Some levels are engaging, but simple and feel more like wasting time, while others step up the curve so quickly that players unfamiliar to RTS (or who just suck at it like me) will lose interest and stop playing, as I did.
(For reference, the particular point I stopped is level 14. I can't get anything to survive, despite disabling mages first)
Aside from this glaring issue, I slaved at this for HOURS. Everything comes together quite exquisitely, thanks in no small part to the soundtrack.
Just missing that little bit to make this a 5-star, but you're damn close. Let nothing stop you.