Abstract. The scope of this research is to identify the concepts that describe cultural heritage monuments and model them with CityGML. CityGML is the most popular data model for storing and sharing semantic 3D geographic data and there is an increasing interest in its use in the Cultural Heritage field. An Application Domain Extension that covers the most important concepts for describing monuments with special focus on the ancient theatres is developed. The INSPIRE data model is reviewed and its integration with CityGML is discussed. Following the proposed extension, a CityGML model is constructed for the ancient theatre of Hersonissos in Crete. To visualize the model, it is transformed using the Generics approach.