This talk delves into the development of highly accurate and open 3D city model data in Japan, which started in 2020. It addresses both quantitative geospatial analysis using publicly available data and qualitative evaluation through 40 use cases. Digital twins, virtual replicas of urban environments, have gained global attention for urban planning, disaster prevention, and environmental simulations. However, geospatial data for digital twins, especially in 3D, has primarily been developed in European and US cities, with limited efforts in Asia. In response, Japan initiated "Project PLATEAU" in 2020 to develop a high-precision 3D city model in CityGML format and convert it into open data. This project aimed to explore use cases and enhance urban policies using digital technology. The study provides insights into the development, openness, and urban data commons in this initiative. It also quantitatively compares PLATEAU data with OpenStreetMap (OSM) data in Japan, highlighting the higher level of detail in PLATEAU but noting the need for regular updates. Furthermore, the talk discusses various applications of open 3D urban model data in Japan, particularly in the realms of smart cities and disaster prevention. It emphasizes the importance of national-level maintenance, integration with open data like OSM, and GIS education in urban planning. The study's data sources are open and will be made available as open data on GitHub, contributing to the reproducibility of the research.