In this paper we explore the iterative design of the Augurscope, a mobile mixed reality device for open-air museum experiences. It allows a 3D virtual environment to be viewed as if overlaid on an outdoor physical environment. While exploring a heritage site, groups of visitors can experience simulated scenes from the past from a dynamic user-controlled viewpoint by moving, rotating, and tilting the device. The development focused on creating an interface to a visualization of a medieval castle as it used to appear in relation to its current, quite different site. We describe the development and application of the Augurscope through two iterative design stages. We discuss the issues revealed through public trials with the first prototype and how they informed the design of the Augurscope 2. The deployment of this second prototype then enables us to offer insights into what makes such a novel presentation device successful in an outdoor museum environment.