I like these types of games. The direction arrows should always be visible so you don't have to look for them to see which way you can turn. The inventory should have all items visible at the bottom.
I do like the pause, play and sound controls on the screen. If not there a menu so you can adjust music and sound effects.
I found a hole to jump in where I could keep on shooting. Score didn't go up but rounds continued and zombies couldn't get me. Still way to dark after color correction.
You need a starting point and a destination for each round.
I HATE games that you have to switch between mouse (to click "NEXT") and keyboard.
This in one of the best of it's kind. Great background art. Can I cut the music and keep the sound effects?
There should be separate buttons to press for HOLD and Gravity Gun. They often do not work when you need them. A great start. Fix a few bugs and this is 5 stars.