Honestly, I think it’d be better if you started by going back to the basics of tower defense games, where the enemies follow a single set path, and then tweaked it as necessary from there. There needs to be some sort of strategy the player can employ to exploit the way the enemies move. They way you did it, there’s no possibility to do that, because the enemies come from all directions and simply move in a basically straight line to the core. That necessarily means that the best strategy will always be to concentrate as much firepower as possible right next to the core, since that’s the only place all enemies are guaranteed to go.