Learning games are an ideal vessel for many kinds of learning content. Playful interaction with the subject matter makes the human mind more receptive and thus learning itself more effective. Well designed games also come with an addictive game-play that makes users want to play the game over and over. This is intended in fun games but it can be counterproductive in learning games as users spend time with content they are already proficient with while they could use this time to learn other less familiar content. Spaced-repetition-learning is a learning approach that helps to improve retention with a minimal number of repetitions. It uses an algorithm based on psychological models to determine when to present a content item. This paper discusses design considerations from both the game-based and the spaced-repetition perspective to explore how the efficiency of spaced-repetition-learning can be combined with the fun and effectiveness of game-based learning.