Just the first of many progress updates. At the moment we've got four maps almost complete: a city map by myself, a gloomy bunker map by Drumfiend, and two maps from Fodder. The programming is coming slowly but surely: zombie AI is finished, and I'm working on coding the interactive objects such as weapon buying zones. I've also got preliminary single player menus working, although there's plenty of bugs to fix.
EDIT: Video demonstrating the work-in-progress zombie leveling. Zombies are at different difficulties based on what wave you are on.