Advanced smart city environments are usually based on a framework composed of several heterogeneous distributed subsystems, including Digital Twins that model infrastructures, assets, and social aspects of the city, as well as analytics and forecasting modules, and IoT sensor networks. Each of these systems must collect and process heterogeneous data formats, and share the results of their elaborations with the other modules. Moreover, they also require interacting with external data sources, such as databases, or other external systems. The integration of these systems into a framework requires dedicated ICT solutions able to support the collection, management, sharing and integration of data, which are also able to facilitate the interactions among them. In this paper, we propose a data lake-based system that has the purpose of collecting, storing, managing, integrating and sharing all the data used by the smart city modules, from both internal and external sources. Moreover, the data lake offers a standardized interface to all subsystems, facilitating their full integration. This system has been successfully tested in a real smart city system in Italy.