Ubiquitous computing architectures enable interaction and collaboration in multiuser 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 multiuser 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, monitoring, remote control and scalability.