Jiuyan Tower, the First Tower of the Great Wall.

Jiuyan Tower is located on the main peak of Huoyan Mountain, 1,141 meters above sea level, southeast of Shiyao Village, Sihai Town, Yanqing District. It was called "Huoyan Dun" in the Ming Dynasty, and later renamed "Huoyan Mountain Tower" and "Huoyan Mountain Watchtower" after Qi Jiguang and Tan Lun requested the construction of a hollow enemy tower. Its military strategic position is very important, located at the junction of the three towns of Xuanfu, Jizhen, and Changzhen. In the early Ming Dynasty, it belonged to Jizhen, and in the 36th year of Jiajing (1557), it was assigned to Xuanfu Town, serving as the junction of the southern and eastern sections of the Great Wall. In the Qing Dynasty, it was called "Yujing North Tower" and "Yujing North Platform." It is commonly known as "Jiuyan Tower" because each side has 9 arrow windows. It is the largest hollow enemy tower with the most arrow windows discovered nationwide so far. It is rare in terms of architectural scale, historical culture, and artistic value, and is known as the "First Tower of the Great Wall."