The proliferation of sensor and actuator networks in the urban environment is directly associated with an increasing volume of data, relating to various aspects of the city. Synchronously, the heterogeneity of data formats, sensory observations, measuring capabilities and accompanying contextual information is substantially growing. Aggregating immense amounts of data feeds from different networks is not necessarily sufficient for providing a better understanding about the processes taking place in the city. Instead, it is essential to extract knowledge patterns that are correlated and integrated in meaningful stories about the urban environment. In light of this new dimension of reality, the OntoPolis© project proposes an open participatory platform with the aim of allowing citizens and policy makers to assess key performance indicators (KPIs), pertinent to environmental conditions, energy consumption and mobility issues. To achieve this, the system links Semantic Web technologies and standard data encodings with diverse sensor networks that employ both technical devices and human agents. By embodying such methods in the urban design process, the project seeks to break new ground in the emerging fields of data-driven urbanism and urban informatics, towards the development of intelligent city environments. This paper introduces the conceptual and technical underpinnings of OntoPolis©, exemplifying the environmental-indicators case scenario.