The concept of Digital Twin (DT) holds great potential for all sectors seeking to monitor, automate, and optimize their processes. This paper focuses specifically on Digital Twin Networks (DTNs) and City Digital Twins (CDTs), offering an analysis of the relevant literature while exploring their interrelationship, which is essential for urban planners and network operators. Although DTN is foreseen as a pillar of future 5G and 6G network architectures, it is often addressed in isolation in the existing literature (i.e., with limited consideration of other domain-specific constraints), even though it constitutes an essential asset in urban environments. In contrast, CDT is often based on the idea of infallible connectivity, which is an optimistic assumption. This study details the benefits and challenges associated with the integration of DTNs into CDTs, paving the way for further research in this field.