The writing says 忠于毛主席, or "loyal to Chairman Mao", 倪阔乐 says. as This has a pretty interesting history.
It is said that it was constructed by Red Guards during the early Cultural Revolution, out of rocks painted with lime. It is a famous slogan, the first part of the "三忠于四无限" movement (three loyal, four infinite), which provides the following slogans:
- 忠于毛主席 (loyal to Chairman Mao)
- 忠于毛泽东思想 (loyal to Maoism)
- 忠于毛主席的无产阶级革命路线 (loyal to Chairman Mao's proletarian revolutionary path)
and the four infinites refers to the degree of loyalty to the above three:
- 无限崇拜 (unlimited worship)
- 无限热爱 (unlimited adoration)
- 无限信仰 (unlimited faith)
- 无限忠诚 (unlimited loyalty)
As the Cultural Revolution ended and its legacy discredited, the writing became obscured by brush. But for whatever reason, in 2008 the area was cleared and repainted, making the writing visible again. It's placed in a very prominent position in the Mutianyu section of the Great Wall, a section that's already special as it's one of the few with merlons on both sides of the wall - very picturesque. Here you can see before/after pictures of the clearing, note that the writing is visible even from far away on the wall, as it's on the inside of a turn:
Photos from http://www.thegreatwall.com.cn/phpbbs/index.php?id=125518&forumid=1&threadid=125489
What's interesting is why the writing was restored, as usually the Cultural Revolution and its artifacts are unpopular even in China. Perhaps after so many years, it's gained historical value. Perhaps it's become popular by Chinese millenials, who have not lived through the Cultural Revolution - it was also a time where leftists like Bo Xilai were at their peak in popularity.