The Citadel of Thang Long is an ancient citadel located in the heart of Hanoi, the capital city of Vietnam. The citadel was built in the 11th century during the reign of King Ly Thai To, and served as the political and cultural center of the Dai Viet Kingdom for nearly 13 centuries.

The Citadel of Thang Long was designated as a UNESCO World Heritage Site in 2010, and is regarded as one of the most important historical and cultural sites in Vietnam. The citadel is comprised of several different structures, including the Forbidden City, the Imperial City, and the surrounding walls and gates.

The Forbidden City was the residence of the royal family, and was the most important structure within the citadel. It was surrounded by walls and moats, and was only accessible to the royal family and their attendants. The Forbidden City was the site of many important political and cultural events throughout the centuries.

The Imperial City was the administrative center of the citadel, and was where the king and his officials conducted the affairs of state. It was also home to several important temples and shrines, including the Thien Huu Pagoda, the Doan Mon Gate, and the Hau Lau Palace.

The walls and gates of the citadel were designed to protect the city from invasion, and were constructed using a combination of stone, brick, and earth. The citadel was heavily fortified, with several layers of walls and gates that were designed to make it difficult for invaders to penetrate.

Today, the Citadel of Thang Long is a popular tourist destination, and visitors can explore the various structures and learn about the history and culture of Vietnam. The citadel is open daily to visitors, and guided tours are available for those who want to learn more about its history and significance.