The description for this on the Jagex site read:
Castle GD, is a complete multiplayer games-suite! Set inside a 3D-rendered castle you can choose to play the other players from a selection of ten games. The games available are, Checkers, Chess, Battlecruisers, Quadlink, Reversi, MysteryMonster, Mahjong, Pairs, Crypt & Dungeon. Features amazing raytraced graphics, texture mapped 3d, chat rooms, league tables and more!