Amazon offers Steam downloads of Castle Crashers for Windows and Mac for $1.49. (Steam charges the same.) That's $14 off list and the lowest price we could find for this co-op adventure.
Want to get your friends involved? Steam offers a 4-pack for $4.49, a savings of $55. (This will provide a download for you, and three giftable downloads.) Steam's deal ends December 21 at 1 pm ET.