The mountains protected the city from barbarians on the plains to the north, west and east, and that was one of the reasons why Kublai Khan established his capital there in 1267, starting what the Chinese call the Yuan Dynasty. The city was then called Dadu. You can still see some ruins from that time at Beijing’s Dadu Ruins Park between the Third and Fourth ring roads north of the city center. The park contains some replica stuff, and an old mud wall that dates from Kublai Khan’s time.
But there is much more Mongolian romance to be found outside of the city in the mountains, where you can combine historical pursuits with some of the finest day hiking in China.
The area around the village of Fanzipai in Miyun County to the north of Beijing is mountainous and wild. There are villages like Fanzipai in the valleys, and you can use them as jumping-off points for hikes into the mountains.