Open source software systems provide a variety of field-tested components offering software development organizations the potential to reuse and adapt such components for their own purposes. The main challenge before achieving the reuse benefits is to acquire a thorough understanding of open source software systems (i.e., the reuse candidates) in order to reason about alternative solutions, to learn about the points where to adapt the system and eventually to decide whether or not to invest into reuse. Manually analyzing even small systems is a time-consuming, complex and costly task. In this paper we present a case study where we analyzed the Apache Tomcat web server supported by a software architecture visualization and evaluation tool and demonstrate how the tool facilitated our comprehension tasks to learn about the architectural means and concepts.