What's this castle all about?

I just joined a game of TF2 to find myself on some sort of castle map, with no guns allowed. What are the rules here, and what weapons are usable?

Solution 1:

Looks like you've stumbled across the Medieval Mode map Degroot Keep.

Medieval Mode is a game mode added to TF2 during the Australian Christmas event with the December 17, 2010 update.

The game play is different with only non-ranged weapons (apart from bows or crossbows) allowed, amongst other changes including "Ye Olde English" for chat, for example:

"it was" gets replaced by "'twas"

Solution 2:

It's Medieval mode, and no ranged weapons are allowed except for bows and crossbows. See the link for the full list.