Go sightseeing to Gubeikou in Miyun, one must go to Nuanquankou and Simatai. 

Nuanquankou is located at the foot of the Great Wall of Jinshanling. It is a small mountain pass. Under the west side of the cliff of the north of the pass, there is a water spring. As the spring lies outside the gateway of the Great Wall, and the water in it is hot, it is called "Nuanquankou". Both in summer and winter, the water temperature of this spring is above 30 ° C with the outflow of dozens of meters and the water temperature unabated. In the winter mornings, standing in the Great Wall overlooking, one can see the warm spring steaming with a lot of mist above. Local villagers often come here to take a bath, the spring water has therapeutic effects on human eczema, arthritis, and skin itching. 

Interestingly, the east and west peeks of Nuanquankou are no more than 200 meters away from each other. The west side is warm spring, but the East side is cold spring. Two springs twist round the mountains, burble and meet at the foot of the mountain. They accumulate into a lake and become a natural reservoir free of ice in winter. 

Nuanquankou is an important component of Gubeikou pass with as its neighbor Simatai which is the stronghold of guarding troops in the ancient times. There are castles built in Simatai, the origin of which has a legend. In Song Dynasty, Panrenmei acted as spearhead with the camp pitched at Chaoheguan, Gubeikou. One superior of him, a captain called Huanglong, garrisoned Caijialukou. Once Huanglong went to Caiheguang, Gubeikou to report to Panrenmei the intelligence of the military. On the way, he passed by Nuanquankou, and met one pair of twin sisters Junying and Junlian in the village. Huanglong saw their two sisters were fair, slim and beautiful just like two flowers. An evil idea suddenly came into his mind. He snatched Junying to the Caojialu camp, then took JunLian to Caiheguan, Gubeikou, and gave her away to Panrenmei. Although their parents begged hard to him, all was useless. Besides, they were cruelly flogged for it. People hated Huanglong, and when Huanglong went to Chaoheguan again, people set a trap in the pass of north of the Great Wall. When Huanglong arrived here, he fell into the pit together with his horse. The horse broke its legs, and Huanglong was beheaded in Caoheguan for delaying the intelligence of the military. The horse died here, thus here is called "Dead Horse Tai". Over a long time, it was wrongly spread and thus called "Simatai" later. So far, outside the castles of Simatai, there still remains the relic of “The Pit of the Fallen Horse”.