Comprehensive overview of Digital Twin (DT) technology, detailing its architecture, applications, and security considerations, particularly within the context of Industrial Internet of Things (IIoT) and smart cities. They establish the DT as a virtual replica of a physical entity, capable of real-time synchronisation, simulation, and analysis for purposes like predictive maintenance, energy management, and urban planning. Several sources focus on the architectural components required for DT systems, including the virtual representation, service interfaces, and the critical IT/OT platform which must incorporate trustworthiness elements such as security, privacy, and resilience. Furthermore, one source explores the critical need for security, demonstrating a novel approach that integrates DT with an Intrusion Detection System (IDS) like Snort to simulate cyberattacks and enhance real-time defence against threats like Denial-of-Service attacks.