Smart Cities are among the most dynamic and rapidly evolving modern environments, driven by the development of new technologies and the fast growth of the Internet of Things (IoT), which enable the acquisition and processing of very large amounts of data. However, accessing IoT assets is proving to be a challenge, as neither formal nor de facto standards to discover connected Things have emerged. Services that provide discovery and access capability for IoT resources are in the rise, but they often adopt service-specific interfaces and authorization mechanisms that hinder the development and maintainability of IoT applications. Low flexibility and interoperability become especially problematic during emergency situations, when responders might need to access resources that normally would not be allowed to access. To address these issues, this paper describes MARGOT, a distributed edge computing platform that supports domain-aware and secure discovery of IoT resources in Smart Cities. Experimental results obtained using MARGOT in an emulated network environment show that our platform can effectively reduce discovery latency and bandwidth consumption under the considered use cases and network conditions.