You did a good job with the art for this game. The idea is quite interesting, but I think I would need to see a more polished game to appreciate it better.
A game that 'reversed the roles' of tower defense very well is Loop Hero, I don't know if you know it, but it's very interesting. Perhaps if you plan to pursue this game idea of yours, it's worth a look for more references and inspiration.
Anyway, I hope you keep developing games. Your art style is really cool and I can see you have potential.