This paper is an attempt to analyse and explain the ‘design decision making’ behind iSpooks, an audio based adventure game for the iPhone. The game utilizes audio as its principle mechanism for driving the gameplay. The design and production team were therefore forced to look at and reconsider the ways in which a mobile phone game works. This resulted in some innovative and previously untried techniques in the design process and technical solutions developed for the project. It also, and perhaps more importantly, addressed and analysed aspects of mobile phone users behaviour in relation to these innovations. This paper covers the research and conclusions that lead to the final game design, and describes aspects such as, sound design, designing for the casual gamer, finding appropriate narrative forms and creating non-visual immersion.