Ubiquitous computing architectures enable interaction and collaboration in multi-user applications. We explore the challenges of integrating the disparate services required in such architectures and describe how we have met these challenges in the context of a real-world application that operates on heterogeneous hardware and run-time environments. As a compelling example, we consider the role of ubiquitous computing to support the needs of a distributed multi-user game, including mobility, mutual awareness, and geo-localization. The game presented here, "SoundPark", is played in a mixed-reality environment, in which the physical space is augmented with computer-generated audio and graphical content, and the players communicate frequently over a low-latency audio channel. Our experience designing and developing the game motivates significant discussion related to issues of general relevance to ubiquitous game architectures, including integration of heterogeneous components, m...