Inside Taihe Dian, Zheng told Obama about the building's history and its architecture and Obama asked about the words hanging on a board in the middle of the hall.

"Jian Ji Sui You," he was told. It meant "emperors should make good rules".

Taihe Dian was one of the largest wooden structures ever built in China and is the biggest hall in the Forbidden City.

At the Palace of Earthly Tranquility (Kunning Gong), Obama peered through the glass into the rooms that were used on the emperors' wedding night.

"It is truly majestic, and a testimony to the greatness and longevity of Chinese civilization," Obama wrote in the official guest book before leaving the museum.

Obama was the fourth incumbent US president to visit the Forbidden City, following in the footsteps of Richard Nixon, Gerald Ford and Bill Clinton.

Asked by reporters what he thought of the Forbidden City, Obama flashed his trademark smile.

"Beautiful," he said.

After a slight pause he added, "spectacular".

US President Barack Obama visits the Forbidden City in Beijing November 17, 2009. [Wu Zhiyi/China Daily]

US President Barack Obama visits the Forbidden City in Beijing November 17, 2009. [Wu Zhiyi/China Daily]