We present a test-bed platform for the iterative design of multimodal games on a mobile phone or a PDA. While our test-bed platform is general to multimodal systems, in this paper we focus on games on mobile devices, since games are intrinsically multimodal, enriching thus the gaming experience and potential for interaction innovations. As part of a player-centered design approach, our goal is to be able to quickly test various forms of multimodal interaction before beginning with the development of the final multimodal game on mobile devices. We adopt a micro-approach for our game test-bed, focusing on multimodal interaction for generic tasks, including moving of an object or a character. Our test-bed facilitates the efficient exploration of the multimodal design space and enables controlled experiments of the designed interaction in-lab and/or in-situ.