Am I correct in assuming that this game will be tile based terrain with 3D assets occupying the tiles?
Yes you're right except that I'm doing everything in 2D because I'm not good enough in 3D for now and I don't think it's necessary in this game.
Either that or you could have smaller, PC controlled bases that can be destroyed in order to gain enough money to build up your own base and maybe make your missiles fly far enough to connect with the enemy base.
This would be awesome but unforunately this would require radial gravity in my engine. For now I just take the two-base concept (one on the upper field-edge and one at the bottom) because this works with axis-aligned gravity. I want the game to be a combination of tactics (where do I place what towers?) and skill itself (controlling manual towers like setting angle and initial missile-velocity, like in Cannon Hill or Worms)
You have limited slots in your base, that you can use for offensive structures (long range missiles), defensive structures (anti-missile guns), and resource generators (factories). The real gameplay is in balancing the three aspects, so you have enough resources coming in to generate attack big enough to get through the enemy defenses, but have room left in your base for long-range missile silos to carry out those attacks, all while keeping everything from being blown up by the other guy with your anti-missile defenses. On top of that essence, you can add tech trees or special units to whatever depth you want.
Limiting space is a good idea I have not thought about yet - Maybe I can add a feature where you can extend your base with additional slots/tiles.
Factories that produce for example ammo are much better than just buying everything. Thanks for this great idea!
What special powers can you buy again after you used it? I thought of something like sending a troop of ships (controllable by the player with arrow-keys or controlled by AI?).
Tech trees are cool and make the game more complex, but this comes later ;-)
Edit: Grammar, Question "how is the player involved in the game" removed because Prinz Eugen already described it