Killing Floor + Defence Alliance 2

Killing Floor is a cooperative first-person survival horror shooter set on the streets of abandoned cities and outskirts of Britain shortly after the failed experiment to create and clone "super soldiers". You and your comrades are members of the armed forces dropped into these places with the simple task of surviving as long as possible by clearing the landing zone of the consequences of a failed experiment!
Co-op gameplay - six players against countless mutants
An upgrade system that allows you to use online achievements to improve your hero´s skills
Over 170 achievements, such as "Respect for Death", awarded for killing 10 monsters that devour the corpses of your teammates, or "Hot Fun" for killing 25 burning monsters with a crossbow
"ZEDtime" slow-motion feature that allows you to see in-game moments in detail even in multiplayer
Single game mode "in one"
Ten types of monsters only concerned with tearing your head off. In addition to jaws and claws, these brats can be armed with chainsaws, machine guns, or even grenade launchers.
A wide range of weapons with more than thirty-three types of weapons - from knives and a fire ax to a shotgun, rifles and a flamethrower
Welding tools, medicines and armor designed to make survival easier
Improvement system. You can choose those that will help you resist the countless crowds of mutants or will simply be useful to your team.
Open, non-linear game world: you are free to choose where and when to fight or retreat. Weld doors to make monsters go around narrow corridors
Full customizability, allowing players to change the difficulty of the level, the number of waves, or even set their own, made up of their favorite monsters
Support for Steam and Steamworks functionality
A set of materials for creating new levels and mods