Nice game.
What I like :
- Music
- Knowing where an enemy spawn before he spawns
- SFX
- Massive attack
What could be improved :
- More juicy (screenshake/particles)
- Not sure we need slots, just resources would be enough
- Difficulty curve : Did not test the Upgrade system, I die before because I'm not enough patient/skilled to run/shoot/run/shoot etc...
Hope it helps