Game sketching is used to identify enjoyable designs for digital games without the expense of fully implementing them. We present Raptor, a novel tool for sketching games. Raptor shows how table-top interaction can effectively support the ideation phase of game design by enabling collaboration in the design and testing process. Raptor heavily relies on simple gesture-based interaction, mixed-reality interaction involving physical props and digital artifacts, Wizard-of-Oz demonstration gameplay sketching, and fluid change of roles between designer and tester. An evaluation of Raptor using seven groups of three people showed that a sketching tool based on a tabletop computer indeed supports collaborative game sketching better than a more traditional PC-based tool.