The remnants of the wall that surrounded Beijing, China

The fortification walls, both an inner wall and an outer wall, surrounding Beijing city were built from the early 1400s to 1553.
The dimensions of the Inner city wall are:
          Length: 24 kilometers or 15 miles
          Height: 25 meters or 49 feet high
          Thickness, at ground level: 20 meters or 66 feet
                            at the top: 12 meters or 29 feet
It had nine gates.  The fortifications included gate towers, archways, watchtowers, barbicans, barbican towers, sluice gates, sluice gate towers, enemy sighting towers, corner guard towers, and a moat system.
The outer city wall had a length of 28 kilometers or 17 miles.
From 1911, after the collapse of the Qing Dynasty, the dismantling of the fortifications began.

In 1965, major deconstruction of the fortifications was commenced to allow for the construction of the 2nd wrong road, and Line 2 or the Beijing underground railway.

 In 1979, the government called off the demolition of the remaining city walls and named them cultural heritage sites. By this time, the only intact sections were the gate tower and watchtower at Zhengyangmen, the watchtower at Deshengmen, the guard tower at the southeast corner, the northern moats of the Inner city, the section of the Inner city wall south of the Beijing railway station, and a small section of Inner-city wall near Xibianmen.

For more reading, go to: https://en.wikipedia.org/wiki/Beijing_city_fortifications

As for our guide's explanation of the fortifications:

Leaving the Square to go to the Golden Mask Dynasty Show, we pass remnants of the wall that used to surround Beijing.

This wall was built in the early 15^th century and was about 24 km long, up to 15 meters high and about 20 meters thick, and had nine gates, one of which still exists today.  In 1965 most of it was removed so that the second ring road and an underground railway line could be built.