Thank you for taking the time to play, I'm glad you had some fun :)
The codes needed to be input into the different keypads that were around the environment. Each keypad is 'hidden' behind a door that is essentially unlocked by killing enemies (killing enough eventually mutates you to a point where the light on your torch corresponds to the new door colour you can walk through). Unfortunately our tutorial was pretty crude so we probably didn't get this concept across very well.
The enemy spawns are tied to player progression in the sense that killing more enemies allows the player to mutate but also mutates the available enemies. It sounds like you may have killed enough that it unlocked an uncomfortable amount of zombies but not enough to unlock all the doors and give you more space to manoeuvre.
We really appreciate the feedback, it's given us some things to work on!