This paper describes the process of rapid iterative prototyping used by a research team developing a training video game for the Sirius program funded by the Intelligence Advanced Research Projects Activity (IARPA). Described are three stages of development, including a paper prototype, and builds for alpha and beta testing. Game development is documented, and the process of playtesting is reviewed with a focus on the challenges and lessons-learned. Advances made in the development of the game through the playtesting process are discussed along with implications of the rapid iterative prototyping approach.