A modder with too much time on his hands has lovingly created a Left 4 Dead Survival mode map that allows the Survivors to defend Helm's Deep - as seen in Peter Jackson's film The Lord of the Rings: The Two Towers.
This map was actually created over a year ago, but has recently come to light via a YouTube video of some friends playing it. Complete from the outer wall to the Deep itself, the map is impressive in scale and even more so in detail. Keep reading to see the YouTube video, or go here to download the map itself.
And no, you can't actually blow the horn. Sorry.