This paper presents an Internet-of-Things software infrastructure that enables energy management and simulation of new control policies in a city district. The proposed platform enables the interoperability and the correlation of (near-)real-time building energy profiles with environmental data from sensors as well as building and grid models. In a smart city context, this platform fulfills 1) the integration of heterogeneous data sources at the building and district level, and 2) the simulation of novel energy policies at the district level aimed at the optimization of the energy usage accounting also for its impact on building comfort. The platform has been deployed in a real-world district and a novel control policy for the heating distribution network has been developed and tested. Results are presented and discussed in the paper.