Although this game's core idea came from ace attorney, you have taken it completely in your stride. Not only that, but in many ways this game is presented better, and more sensical than the game it's taken from!
Also, once a large fort is built, later stage become very easy to fend off. I think the Hard levels need to be made significantly harder, whether you introduce a new enemy or you just increase the number of enemies.
On Red Dawn, for the last 5 or so waves, even quadruple the enemies... It's really way too easy.
Very fun :D
The only hing I'd suggest is allow players to edit their defenses during the attacks.
It wouldn't particularly unbalance the game, and in the later stages, it would make upgrading walls far less tedious and time wasting.
Doing lvl 7 now, it's not too hard. Just build a heap of reactors and don't open a route northwards until you are really ready.
Also, at the start, if you build to quickly you will run out of energy, and all construction will slow to a snails pace. So be mindful of that.
Hope I've helped.