Castlefall (Castle of the Devil + Spyfall) is a party word game for usually six to ten people, preferably an even number, but still playable with four or five (or even three!). I'm not totally sure who invented it but believe it was stolen from ET.
Castlefall is played through a website (bpchen wrote one, the code is on GitHub; and it's hosted on his website). Players should load the site; they will each receive a word list, which is the same across all players in the round, and one word from the list. The players are in two equally-sized teams (or almost-equally-sized if there are an odd number of players); each player on a team has the same word, and the two teams have different words, but players don't know who else is on their team.
Players begin discussing, with the goal of trying to figure out who else is on their team or what the other team's word is, until somebody declares victory (usually signified by clapping loudly, since earlier declarations take precedence). There are two ways to declare victory:
- Choose N players (including yourself) and claim that they are on the same team. N is usually 3 for 6- to 8-player games and 4 for 9- and 10-player games, although there's some player choice here. Nobody else can declare victory with this method after you have done so. Start a one-minute timer, and continue discussing; if nobody else declares victory using method 2 after one minute has elapsed, the round ends, and you (and your team) win iff your declaration was correct.
- Guess the other team's word. The round immediately ends; you and your team win iff your guess was correct.
Note that you always win or lose with your team (the set of people who had the same word as you did).
The usual strategy is to give clues about your word that are recognizable to people on your team who are trying to fit that word with the clue, but not so obvious that your opponents will be able to figure out your word from the 17-or-so other options. Castlefall is all about striking this balance. Note that you can react to clues that you don't actually recognize to trick them into thinking you're on their team. You can also give clues about other words, perhaps words that you suspect are the other team's, and see if anybody else reacts to try to guess the other team's word. (The technical term for this, according to floorpi, is "trolling".) This runs the risk, however, of tricking somebody into thinking that that other word is actually your word and declaring victory on it; that somebody may or may not be on your team.