A musical composition is like a game in that the rules and parameters controlling the structure of an aesthetic experience are devised prior to its realisation in performance. In a musical work, the composer specifies how these rules and parameters should be realised over time and an ideal performance is a manifestation of the composer’s artistic intentions. In a game, however, it is the player who determines its trajectory. In light of this, a game experience can be viewed as an exceptionally rich data source: a product of the designed dynamics of a game world and a player’s traversal, or interpretation, of this world. This product has rich musical and experiential possibilities. While many new electronic games prominently feature music, very few attempt to produce a genuinely original musical experience derived from the manner in which the game is played. Game play as a data source is left untapped. This paper argues that an interdisciplinary understanding of game design theory provides the tools for analysing and interacting with game play as a data source; music can be linked to the underlying design of a game rather than merely its visual representation. A composer can design games in which a player’s sense of musicality forms part of the game rules; games in which a player’s sense of game play shapes a musical experience. The author’s Battle Metris is presented as a game which allows a new type of musical experience for observers and participants which is a mixture of the individual participant’s musicality and sense of game play.