The moat surrounding the Beijing Forbidden City is called the Tongzi He(筒子河), also known as the Inner Moat. It is part of the inner city moat system of Beijing and serves as a protective barrier for the Forbidden City. The Tongzi River is approximately 3.5 kilometers long and has a width of 52 meters. It divides the area around the Forbidden City into four sections: northwest, northeast, southwest, and southeast, with the Meridian Gate (Shenwu Gate) and the Gate of Supreme Harmony (Wumen) serving as the north-south axis and the East and West Prosperity Gates (Hua Gate) as the east-west axis.
Beneath the road surfaces of the East and West Prosperity Gates and the Meridian Gate, there are small culverts that connect, each with a cross-section of approximately 50 centimeters by 60 centimeters. Inside the Forbidden City, there is a stone-built Ming Dynasty canal that connects to the Tongzi River in the northwest. It runs from west to east, passing through halls such as the Hall of Martial Valor (Wuying Hall), the Gate of Supreme Harmony (Wumen), the Hall of Literary Glory (Wenhua Hall), and west of the East Prosperity Gate, it joins the Inner Tongzi River on the west side of the southeast corner, known as the Inner Golden Water River.