Good gameplay! Very fun! Way too easy, though.
One of the early levels gives way too much money for very little work and can be easily completed with just the basic gun (I think level 3 but I can't remember now). The level gives more than half of the money that the boss level gives, and finishes in half the time. Compare that to other levels that are long and need proper gear that give less than 1/6th of the money compared to the boss. Even not exploiting that high money early level, I beat the game without dying and only buying one gun.
There are a few coding oddities, too, but they are minor and you will learn to avoid them. Overall excellent fun game!