I like the idea, but I think there can be some improvements.
Here are a points I'd like the bring up:
-If your units attack the enemies are the enemies going to ignore them or will they fight back? If they fight back, will they attack your walls? If you decide to make them fight back, it will feel more like a survival game than a TD/Hero Defense (unless done carefully to achieve a hero defense).
-Gold should be the only resource. It will put more weight on the choices the player makes, and will ultimately decide whether or not they can survive.
-How will the training ground work? Right now it sounds like each unit is a hero-type unit and will actually level up by training (which would be a cool idea until you realize how many hero units that will leave you with). Maybe you could have units constantly spawn at the "training ground" to attack to your main base (no mazing, just the enemies and your army), thus forcing the player to multi-task and divide their army effectively. This will, however, require some balance so it does not overwhelm [new] players.
-Not sure about the time limit as opposed to numbered waves. I'll not get into detail less I get really complicated with my above point, but how do you see the main battle working out?
Other than detail polish, this is a really neat ideal. I could help out with triggers (or terrain), but I am NOT doing units again. I tried to do a TD once, but the mix of complicated triggers (and the lack of experience with them years ago), dozens of units and hundreds of unique towers bogged me down until I didn't go on (my idea was very unit/tower idea heavy; more so than most TDs).