3D geodata are more and more available as well as realtime visualization possibilities with free three-dimensional viewers such as Google Earth. This implies a growing demand of 3D city models, which are 3D representations at the scale of the city. Despite their intended wide range of applications, such models cannot be used for many urban tasks as they cannot represent the urban information associated with these tasks. On the contrary, ontologies have proven their capacity and usability in the representation of information and knowledge of various domains. In this paper we will present, on the basis of case studies, how ontologies can overstep the semantic limitation of 3D city models and how ontology-based models can be interconnected thus increasing the interoperability of urban information.