This paper presents deep decarbonization strategies for city-level energy systems. Helsinki city is used as a case in the analysis. The strategies are mainly based on extensive electrification employing renewable electricity, storage, and sector-coupling strategies. We perform energy, economic, and resilience analyses for the different cases. An energy balance model with 1-h resolution is used to optimize the energy system on macro-scale, while a MILP-algorithm is used for micro-level optimization of operation of individual plants against different criteria. The results indicate that a zero-carbon energy system is feasible by 2050, but it would also require coupling to the exogenous energy system (national electricity market) to balance mismatches. Power-to-heat coupling, or storage alone would not be adequate. As an example of system dynamics limitations, with a wind power capacity of 1.5 GW corresponding to 56% of the annual electricity demand in Helsinki, 90% of the wind electricity can be used locally in the different sectors, but the rest needs coupling to the exogenous market due to mismatch and plant limitations. The decarbonization strategies with increasing variable renewable energy production generally improve the resilience of the energy system, but with some concerns to adequacy of peak production and electricity dependency of heating.