The idea is very cool! It might be a bit too ambitious for a game jam, but hats off to you for giving it a try!
There are quite a few feedback elements missing that would have made the experience clearer and more enjoyable, but as I mentioned, it was a complex project to tackle alone in such a short time, so it's completely understandable.
The one thing I think you should have really pushed to include was a money counter, that would have made a significant difference!
I believe that, with further development, this idea could become truly interesting if applied to a multiplayer co-op mode, where one player defends the walls and the other manages the constructions. I think that would be a blast!