Next week Valve will release the first major content update for their zombie-themed shooter Left 4 Dead. One part of the update is the Survival mode and this week's update on the game's official web site (written by Valve's Kerry Davis and Scott Dalton) deals with how the team came up with it's design.

Survival Mode will let players see how long they last against a wave of endless zombie attacks. David and Dalton talk about how hard it was to ramp up the diffiiculty for each zombie wave in the mode. They state, "It might seem that just gradually increasing spawn rates and numbers of infected will create a nice smooth curve, but it turns out to be much trickier than that." You can check out the site to find out how the development team managed to find a balance between making the mode challenging but still fun to play.