We decided to make a custom game engine for this project. The first days the game looked like this and had no real gameplay.

A bit further along in development we started using assets for our character, got animations working and multiplayer networking was being worked on

Zombies were eventually added. At first they would just stand in place and shake. They did not have any animations and they did no harm to the player.
World generation was added to the game. At first it was entierly client side and would make the user stutter. It was slow, and prone to cheating.
Fog was added to the game in order to hide the world loading, and it was optimized for better performance, it was also made entierly server side to both offload the clients, and to prevent cheating.