There are many Tianmen Mountains in China, one of the most well-known is located in Zhangjiajie, Human, and I've visited it during 2023 Spring Festival.
Zhangjiajie is famous for it beautiful natrual sights, and Tianmen Mountain is ranked the second from my personal opinion, it is very very large for supporting tourists to enjoy the view, and the door to the heaven(天门) is a huge one seen from the bottom.
Miyun is known for its water reservoir(密云水库), which is surrounded by lots of mountains, so people can climb mountains there.
Tianmen Mountain in Miyun is a little like its companion in Zhangjiajie in the door to heaven, however it is much smaller than the latter, its scenic spot is small, its mountain is small, its walkways is narrow too. And its door to heaven is a little vertical, which makes it differs from its cousins.
One spectacular spot for MIyun Tianmen Mountain is people can take a birdlook of Miyun water reservior, scattered in different parts of Yanshan mountain range. It looks like many pearls stringed in the green trees and mountain belts. It is very very splended.
I like this trip very much.