I did start to work on a simple Multiplayer
It will be a simplified version of the game. I guess there will be one floor, where you can buy all of the weapons and upgrades and also you will be able to pay for upgrading and sending zombies to your enemy.
It will be a 1vs1 mode. You won't see your enemys base or your enemy itself.
It will be just to defend against each other. First player who kills the other one with zombies will win.
I am really at the basics of this. I started to learn how to create a lobby with steam. I will keep you in the loop! I can not promise if I will get this to work but I hope so!