## 5 Conclusions  
In this paper, a VR-based Digital Twin solution to optimize the management of smart city services is presented. The system, whose concept test is developed at the IoTMADLab facilities in Madrid, comprises various urban elements equipped with IoT devices, facilitating seamless integration between the physical and digital worlds, enabling real-time data collection, simulation, predictive analysis, and control functionalities. The physical world component of the Digital Twin infrastructure encompasses a diverse array of IoT-enabled urban elements, such as streetlights, parking lots, waste containers and building facilities. These devices provide critical data on various parameters such as illumination, occupancy, filling level, environmental conditions, and energy consumption. The IoT platform, based on Home Assistant, serves as the backbone of the Digital Twin infrastructure, facilitating communication between IoT network (gateways and management systems) and the virtual world. Leveraging RESTful APIs and MQTT protocols, the platform ensures seamless data retrieval and command transmission, enabling real-time interaction between the physical and virtual worlds. Integration with InfluxDB database and visualization tools like Grafana enhances data analysis and visualization capabilities, empowering users to gain insights into IoT ecosystem trends, patterns, and anomalies. The virtual world component offers a detailed 3D digital representation of the IoTMADLab environment. Geographically precise and visually realistic, the virtual model enables users to navigate and interact with IoT devices seamlessly using VR headsets and controllers. Optimization techniques ensure smooth performance and immersive user experience across different hardware configurations, while OpenXR API compatibility ensures platform-agnostic support for leading VR headsets. The smooth operation of such solutions, which integrates different technologies (IoT, DTs, and VR), require the use of an IoT architecture based on open and standard protocols. The IoTMADLab proposed a reference IoT network architecture, which serves as a practical example of how these technologies can be deployed to create cross-domain smart city services. It facilitates direct connectivity between devices from different manufacturers and municipal services, promoting interoperability and efficiency in IoT deployments. Moreover, the DT concept, integrated with VR technology, offers intuitive interaction and visualization for monitoring and controlling urban assets. The novelty and value of our approach lies not in the individual technologies per se but in their integration at the city-wide level. This endeavor actually poses a multitude of complex challenges that demand innovative solutions. Firstly, achieving seamless integration of IoT, Digital Twins, and VR across an entire urban ecosystem involves navigating a highly fragmented technological landscape. Cities are complex entities with legacy systems and infrastructure that must be retrofitted with new technologies. Ensuring compatibility between old and new systems, while also maintaining the flexibility to incorporate future advancements, requires an understanding of both the technical and socio-economic dimensions of urban environments. Another major challenge is the creation of a digital twin or a federated group of digital twins that could accurately represent the working of each urban service. This involves not only the technical difficulty of modeling complex urban elements and behavior, but also the logistical and organizational challenges of coordinating between multiple stakeholders, including government areas, private companies, and citizens. Achieving a comprehensive and coherent digital twin that can be effectively used for simulation, visualization, and decision support in VR requires a concerted effort. Lastly, ensuring the system's sustainability and scalability presents another layer of complexity.