The ancient city is in Pingyao county, Shanxi Province. The city wall was first built during the Zhou Dynasty. Since 1997, it has been listed as a World Heritage Site. Now Pingyao city is the only one in China completely protected as it was hundreds of years ago. The perimeter  of this square city is 6.4km long and the city wall is the earliest and largest city wall in China. It is 12 m high with width of 5 m. The wall was built with earth and covered with bricks. Outside the city wall there is a river, 4 m wide and 4 m deep. There are several gates of the city, two each on the east and west and one each on the south and north side. All the gates have two doors each. This has given the city the name "Turtle City" with the two gates on the south and north standing for the head and tail of the turtle and four gates on the east and west as the four legs. The doors on the south and north stand opposite each other, like the head of the turtle extending out and two wells just beyond southern gate are like a turtle's two eyes. Located on the trade route between Beijing and Xi'an, Pingyao developed into a merchant center where the local people set up the nation's earliest banks. These banks were the first in China to use checks. Several old courtyards have been turned into museums, most of which were homes and offices of Pingyao's old banks. One of the most unusual aspects of a visit here is realizing that this place was very important to China's financial  history. What does the passage mainly talk about?
The ancient city--Pingyao. China's ancient financial center--Pingyao. The city wall of Pingyao. The museums in Pingyao.