Apocolypse Game

Apocalypse Game is an Open Source fully moddable  post- apocalyptic adventure where you build, collect and fight! Construct buildings, tools, or scrap them to make something far more useful! Play alone, on LAN or Online with friends- and enemies.
Make your own mods!

But most importantly....

Beware the Watcher.

Requirements:
Windows 7 + 64-Bit
80MB hard drive space min

A copy of the open source engine's code is here:
https://github.com/minetest/minetest/tree/stable-5