It used to be a thoroughfare from Tainan to the Feng- Shan area. It was originally built with wood and rebuilt with concrete and bricks in 1788. The moon-shaped external wall was added to it in 1835 to strengthen the castle. It was called an“Urn City”because it enables the enemies to be easily captured once they enter the castle as in the old expression“as easy as catching a turtle in an urn”. Currently, the graveyards have been removed and schools have been built on the land.