Cổ Loa citadel is also known as the snail citadel because the External and Middle ramparts meet in the south next to the Hoang River. From there, the two fortresses followed the riverbank, turned to the east and west, then went north, far away from each other and never met again. Therefore, it is called a spiral or snail-shaped citadel.

It is at the place near the south gate, close to the Hoang River, that there is a well connecting the water to the river, that is the "snail mouth" or "Loa khẩu".

According to the legend, after several failures in building the Cổ Loa citadel, so the king prayed to Huyền Thiên Trấn Vũ. Huyền Thiên Trấn Vũ sent Kim Quy God - the golden turtle that became Thanh Giang messenger to help An Duong Vuong defeat Bạch Kê Tinh (White Rooster spirit) in Sái mountain. Thus the citadel stood firm. The job was completed, the Golden Tortoise gave the King a claw to make a "magic crossbow". The place where the Golden Turtle shows up is Loa Khau. Later, a shrine was built near Loa Khau to worship the god Kim Quy.

The shrine is located close to the present market Sa, about 20m from Loa Khau. The shrine is simply built of bricks and mortar, including 1 compartment with 3 sides of wall, arched door, altar, area of ​​about 4m2.