This book explores the emerging paradigm of Smart Digital Cities, where digital infrastructure and intelligent technologies converge to transform urban environments. It presents a structured and practice-oriented framework that addresses the evolution of smart city concepts, the integration of next-generation technologies such as AIoT, GPT, and AGI, and the design of scalable, multi-layered urban solutions. Organized into three parts?foundations, applications, and future outlook?it introduces core theories, cross-sector use cases, and forward-looking strategies including cloud-edge-device collaboration, data-driven decision-making, and industrial-urban integration. Illustrated with system models, architecture diagrams, and real-world case studies, the book emphasizes practical innovations while offering strategic insights for policy and technology development. It is a valuable resource for researchers, engineers, planners, and graduate students engaged in smart city design, digital transformation, and sustainable urban development.