It's a recreation of Helm's Deep from the new Lord of the Rings movie. It's got a lot of monsters, and it will slow down your system.
"This is a map I made after playing a Helm's Deep map for Counter-Strike. As far as looks go, I based on pretty much entirely on the CS map (which was pretty accurate as far as I know). In terms of gameplay, don't expect the marines to kill everything for you, you still have a lot of killing to do yourself. Also, the level will slow down at a few points, so be warned. If you have a lower end machine it may beg for mercy if you try and play this."
You need ZDoom 2.0 to run this. Not sure of the oldest version that'll run it, so just use release 42 which you can get at http://zdoom.notgod.com/lars (it's a .cab file).