This book aims to help readers learn about these unique and creative structures. In addition to well-known citadels recognized by UNESCO, such as the Imperial Citadel of Thăng Long, the Citadels of Hoa Lư, Huế and Hồ Dynasty, this books seeks to introduce many others. These were not only significant fortresses in the struggles against aggressors but they are also valuable from a cultural and architectural perspective. Moreover, the ruins of old citadels are evidence of cultural exchange and development of human civilizations. Therefore, they greatly contribute to our learning about the history of mankind.