Software visualization is a popular program comprehension technique used in the context of software maintenance, reverse engineering, and software evolution analysis. While there is a broad range of software visualization approaches, only few have been empirically evaluated. This is detrimental to the acceptance of software visualization in both the academic and the industrial world.

The comprehensive understanding of a large software system is a daunting task because of the sheer size and complexity that such systems exhibit. In this context software visualization is a widely used approach, since well-conceived visual representations allow one to spot patterns. The large majority of visualizations use 2D representations, because they are easier to construct, navigate, and interact with. 3D representations usually exploit the 3rd dimension as an additional means to encode quantitative values, which is dismissed by many as a too small benefit in the light of the added complexity in terms of navigation and interaction. We argue that a well-constructed, interactive, and easily navigable 3D visualization can greatly help in program comprehension tasks by supporting habitability. Habitability transmits to a developer the notion that a software system is a physical space with strong orientation points. This can give developers the feeling of being "at home" in a system. We propose a 3D visualization of software systems hinging on the city metaphor. It is useful for program comprehension because it leads to clarity about the overall structure of a system. We apply our visualization technique on two large systems and discuss its benefits and drawbacks.