A multitude of requirements must be taken into consideration to ensure the safe and efficient integration of crewed and uncrewed airspace users in a future Air Traffic Management (ATM) system – in particular for flights in urban areas. These airspace users can be very diverse and, in addition to Visual Flight Rules air traffic, also include personal air vehicles (PAV), cargo UAS, parachutists, weather balloons and prioritised air traffic such as rescue helicopters. In addition to the aspects of different designs, performance and capabilities, a U-space concept must also take into account additional requirements with regard to protected flight areas and favoured, risk-minimised flight routes. In the City-ATM project, a concept for future air traffic management in urban airspace is being developed, which will enable the safe and efficient integration of new air transport vehicles such as UAVs and air taxis. This includes the definition and validation of operational and technical concepts for airspace management, information provision, traffic flow control and monitoring, as well as basic concepts for the communications, navigation and surveillance infrastructure. Based on these concepts, a simulation and demonstration platform for urban ATM will be developed in City-ATM. This includes the development of an operational concept, the definition of requirements and general conditions for the future ATM system, as well as considerations regarding the technical feasibility of an aircraft that is able to operate safely in these environmental conditions. The target system will enable and support flight testing using the City-ATM in real airspace.