There is a story about Tongwan City within the territory of
Wushen Banner of Ordos City. Elianbobo, king of the Great Xia
Kingdom, has a talented elder sister, who could make soldiers by
scattering beans. She helped his younger brother build Tongwan City
and paved an 800-li-long Zhuanbing Cave under the city that could
lead to Yan'an. After that, she lived in Yan'an. Upon her
departure, she told her younger brother: "if enemy attacked you,
you can knock the bronze bell that hangs in front of the cave's
mouth three times and I will send soldiers to you."
Helianbobo was suspicious and thought that his elder sister was
playing tricks on him. Once day, he knocked the bell suddenly.
Later, thousands of tens of thousands of horses and soldiers came
to the cave's mouth and covered the field on both sides of the
Wuding River. When seeing it, Helianbobo laughed and said: "Haha, I
am playing! What are you doing here?"
Several years later, enemies surrounded Tongwan City. Helianbobo
became flustered and knocked the bell in a hurry. However, his
elder sister didn't send any soldiers to him. Several days later,
the city was taken over by the enemies and Helianbobo became a
captive.