"Castle Defense" ist ja englisch. Daher gehts jetzt englisch weiter! Ha!
Okay, okay. Snarkiness aside.
The idea is nice! I think that exists somewhere already? And even if not exactly this, you will have some pretty well-made competitors, so maybe you shouldn't expect too big an audience for this - which is heartbreaking, given how much time this is going to take. Because this is going to be terribly difficult to implement and create.
I mean it. Multiplayer? Difficult. Asymmetric multiplayer? Even worse! Vehicles and Airships? A nightmare to code!
Look, I know this is discouraging. Sorry. I'm just saying - aim a little lower. Try something simpler. Then, when you have that finished, maybe then think about tackling an asymmetrical multiplayer game. But thats not an easy project (also not easy as a side-project).
If you decide to keep going - okay! All the best to you. But please consider making something simpler. Or stick to your main project - side projects have that tendency to destroy main projects with its siren call.