Mobile technologies offer the opportunity to embed learning in a natural environement. This paper describes the design of the MobileGame prototype. MobileGame explores the opportunities to support learning through an orientation game in a University setting.The paper first introduces the scenario and then describes the general architecture of the prototype. The second half of the paper focusses on requirements that have evolved during the design, implementation and testing of the prototype: Supporting work on the move poses difficult interface questions, the acuracy of current outdoor and indoor positioning systems is still problematic and the game requires near real-time response time. This requires a distribution of functionality and data between the server and the client and a careful design of the interface.The success of the game heavily depends on a motivating design of the game itself (i.e. the setup and its rules). A surprising number of user roles surface once the game is implemented in a natural enviroment.