In Castellan, two players work together to build a castle. Finely-detailed wall and tower pieces link together to form courtyards, and the player who finishes a courtyard claims it with a Keep, scoring points for that courtyard equal to the number of tower pieces surrounding it.
Your cards determine what pieces you get each turn . . . but you decide where to put them. If you're clever, the pieces played by your opponent can end up scoring for you!