Ontologies make it possible to understand, analyze, exchange or share knowledge of a specific domain and therefore they are becoming popular in various communities. However, ontologies can be very complex and therefore visualizations can support users to understand the ontology easier. Moreover, graphical representations make ontologies with their structure more manageable. For an effective visualization, it is necessary to consider the domain for which the ontology is developed and its users with their needs and expectations. This paper presents the development process of Knoocks (Knowledge Blocks) - a visualization tool for OWL Lite ontologies - which was implemented with the help of the human centered design process. The presented case study underlies the importance of repeated usability evaluations during the development process to identify weak points of the design and missing features which are relevant for the intended users.