Thank you for your suggestion! As for the door, it is not for the player to walk through. It is an entry point for the knife zombies to get into your laboratory. As the waves progress, more entry points are created through which the knifes enter the lab. The knifes are spawned randomly at each of the doors/holes. You just have to defend the lab a.k.a. heal the zombies.
Maybe in a future version, additional levels or rooms would be added but for now there is just this one room for the player to defend.