That is a really nice game, every upgrade is fairly easy to find out how it works, the eye puzzle took me some time to figure out but has the nice aha moment (but I bet it could be kinda easy to miss, even with the upgrade right above the answer).
The boss battle has a good difficulty, being challenging even with all the upgrades and I bet it is doable without the gun upgrade, just longer. But I never found out what the portals do.