Semantic 3D city models are increasingly applied for a wide range of analyses and simulations of large urban areas. Such models are used as a foundation for the development of city digital twins, representing with high accuracy the landscapes and urban areas as well as dynamics of the city in terms of processes and events. In this context, this paper presents a 3D city model, which is a starting point for the development of the digital twin of Sofia city. The 3D model is compliant with CityGML 2.0 in LOD1, supporting the integration of the buildings and terrain and enriching the buildings’ attributes with address information. District Lozenets of Sofia city is chosen as a pilot area for modelling. An approach for the 3D transformation of proprietary geospatial data into CityGML schemas is presented. The integration of the buildings and terrain is an essential part of it since the buildings often partially float over or sink into the terrain. A web application for user interaction with the 3D city model is developed. Its main features include silhouetting a single building, showing relevant overlay content, displaying shadows and styling of buildings depending on their attributes.