The design and development of future smart city services will usher with unprecedented creativity, a plethora of functionalities and conveniences to the urban communities. The success of smart city services depends on the assurance that security is maintained and protected to meet community expectations. However, due to the variety of components involved in the design of smart city services and their complex interactions, it becomes challenging to properly assess the overall security of the system throughout the service life cycle. This paper presents a holistic approach to model the security of smart city service infrastructure at a higher level of abstraction. To match realistic security deployments, the proposed model is based on multiple tiers of defense that includes defense at component level, system level, and security operation center. For a given system configuration and component vulnerability vector, the model assesses key security parameters for a variety of attacks. The proposed model is generic and can be used for studying the security of various smart city infrastructures, deployment configurations, and attack vectors.INDEX TERMS Smart city security, threat analysis, probabilistic model checking.