Great entry!
If you have am object that manages spawn rates and level of enemies I recomend to do some levels (from easy to HELL) so you let players learn your game, it's requirements, and the things they should keep in mind.
It felt like Starcraft when I had to manage that every slot is making something and every threatened line is being defended. Congratulations for reaching that that mood!
To conclude, cool entry, cool game, very good design, art & sound. Study difficulty curve.