This paper presents the design and deployment of Polar Defence, an interactive game for a large public display. We designed this display based on a model of "users" and their interactions with large public displays in public spaces, which we derived from prior work. We conducted a four-day user study of this system in a public space to evaluate the game and its impact on the surrounding environment. Our analysis showed that the installation successfully encouraged participation among strangers, and that its design and deployment addressed many of the challenges described by prior research literature. Finally, we reflect on this deployment to provide design guidance to other researchers building large interactive public displays for public spaces. Categories and Subject Descriptors H5.m Information interfaces and presentation (e.g. HCI), H5.2 User interfaces-Graphical user interface. General Terms Design, Experimentation and Human Factors. Keywords Interactive large public di...