The idea of Computer Supported Collaborative Learning (CSCL) is being investigated for more than twenty years. Since a few years, game-based approaches like video games for learning (Serious Games) offer new fields of application. The combination of game-based learning concepts and collaborative learning may enable new, game-based application areas of CSCL, like collaborative multiplayer Serious Games. Designing such games, however, is very challenging as it requires to take into account traditional single player game design concepts, concepts for multiplayer game design, and concepts for Serious Game design simultaneously. Only very few examples of such games exist today. In this paper we describe an approach for the design of game-based collaborative learning scenarios using multiplayer Serious Games. Our approach aims at combining design concepts from the fields of collaborative learning and (multiplayer) game design. Our approach takes into account the requirements of traditional single player games (fun, narration, immersion, graphics, sound), challenges of multiplayer games (concurrent gaming, interaction) and Serious Game design (seamless inclusion of learning content, adaptation and personalization). Furthermore, requirements of collaborative learning are considered, like group goals, positive interdependence, and individual accountability. Our design concept was used to create a collaborative 3D multiplayer game fostering collaborative behavior as a foundation for game-based collaborative learning in small teams. We performed a user study with eight gaming sessions and a total of 23 participants. Results showed that the game enables a collaborative gameplay and fosters collaborative behavior. This may allow us to use a game-based CSCL approach to combine the advantages of game-based learning with those of collaborative learning in future.