I wasn't joking about this, though. It's really fun,and about 30 minutes long. Every 5 minutes(or less) or so a scripted event makes you move back into a different part of Helm's Deep. Like, at 5 minutes the wall you start out on breaks and you have to keep the opening covered by fire, while mowing down the zombies coming through with the chain guns.
I've never made it past 10, but if you have defibrillators, you can teleport the bodies of people who died earlier to later parts of the map. I've also seen this map run with the 8 survivor mod.
Also, there's music and NPCs to help you.