To copy a response I wrote earlier:

Because Premier Zhou Enlai personally protected the forbidden city and ordered army units to be stationed outside to prevent ransacking by the red guards. He ordered this in 1966 after red guards attempted to force their way into the site by ramming the gates with a truck. During most of the cultural revolution the palace was sealed and entrance denied until it was reopened for Nixon's visit. Without his intervention, it is most likely that the site would have being significantly damaged or destroyed.

From Mao's Last Revolution By Roderick MACFARQUHAR, Michael Schoenhals, page 118-119

By the end of the Cultural Revolution, 4922 of the 6843 officially designated "places of cultural or historical interest" in Beijing had being destroyed, by far the greatest number of them in August-September 1966. The Forbidden City (Palace Museum) escaped only because Zhou Enlai got wind of a planned Red Guard Attack. On August 18 he had the gates closed and ordered the Beijing Garrison to send troops to protect it; on August 28, he told representatives from a student umbrella organization, the Capital's Universtiies and Colleges Red Guards' Headquarters (HQ) (later known as No.1 HQ) led by the daughter of Wang Dongxing, that the Forbidden City, the Great Hall of the People, broadcasting station, newspaper offices and airfields were absolutely off-limits.