Most educational or training games, also referred to as serious games, have been developed without an underlying design theory. In order to make a contribution to the development of such a theory, we present the underlying design philosophy of Levee Patroller, a 3D first-person game used to train levee patrollers in the Netherlands. This approach stipulates that the design of a serious game is a multi-objective problem where trade-offs need to be made. Making these trade-offs takes place in a 'design space' defined by three general boundary criteria: 1. fun (game), 2. learning (pedagogy), and 3. validity (reality). The various tensions between these three criteria make it difficult to 'balance' or create harmony in a serious game. We illustrate this process with a discussion on the design of Levee Patroller. In addition, we translate the aforementioned general design criteria into a number of concrete design requirements for serious games.