Digital game-based learning is a widespread trend in education. Nevertheless, many educational games fail because their designers concentrate on achieving learning goals instead of creating an enjoyable gaming experience with a balance between learning success and fun. Traditional game design processes including playtests are seldom used in the development of educational games. For this reason, in this paper playtesting is used to iteratively improve a game-based learning application in two revisions. Gaming experience is measured with the EGameFlow scale. The results show a significant improvement in almost all measured dimensions.