An interesting exposition of the topic, good work. Played briskly, here's what I noticed separately:

1) Change the layers, so that the slot machines do not block the enemies

2) Disappearance of corpses of enemies after a while

3) Add recharge to the Kalashnikov gun, because I knocked him out with the first roll and further there was no motivation to twist and not to remove his finger from the left button of the mouse

4) From the paragraph above would also add rebalancing weapons, so that the coolest does not fall on the first roll and was gradation by level

5) Add time before spawning enemies after transition to a new level, I have a few races lost during these transitions, not having time to react to the enemy

6) Increase the bonus for increasing the number of lives to give not an empty heart, and immediately filled

7) The location of slot machines I either randomized, or would have placed in opposite corners of the location, to motivate the player to move more in the location.

8) I would give out some kind of reward for the killed boss, in addition to the appearance of the door)

9) Make a points system where for each killed unit some amount would be added

Don't get me wrong, my English isn't the best, I enjoyed the game, and these are more clarifications and details that would keep players even more engaged in your project. Good luck!

Pretty fun and satisfying!