The EU project SmartResilience aims to develop methods for practical assessment of the resilience of smart critical infrastructures (SCIs) in urban environments. The paper will describe an approach applied in the case of the energy supply system of the city of Helsinki, Finland. One selected scenario involves a cold winter period challenging the supply and performance of the city district heating network that provides heating for most buildings of the city. A fire in an underground fuel storage initiates an escalating chain of events to reduce production capacity first in the adjacent plant and then in another heating plant of the network. Simultaneous extended period of high demand during the cold spell will trigger regional cooling in the district heating network, so that elderly and other vulnerable citizens may need to be relocated for safeguarding when they cannot rely on their own back-up alternatives. The scenario implies cascading involvement of city infrastructures and actors beyond the energy supply system. The details will affect the essential aspects, phases and indicators of functionality and resilience, and the effort to improve or optimize resilience. The tools developed for the project have helped to describe, visualize and quantify resilience for these purposes. The results and implications are discussed from the point of view of other plausible threats from extreme weather events or interdependencies in the evolving regional and European energy systems.