Base Defense is a single/cooperative mod based on horde-style system.
It's awesome, you should try it ;)
Today I'm started to work on update for Base Defense.Dec 4, 2012 News
Hey guys! Finally I released the last version of base defense.Oct 31, 2012 News
What you'll see in next version of Base Defense and what I'm working on.Aug 18, 2012 News