Ho Hoan Kiem or Lake of the Returned Sword was once a part of the Red river (song Hong). Through thousands of years of changes in the geography, the lake moved eastward to its present position many kilometers from the river. The lake was once called Luc Thuy or Green Water because the water was green year round. In the fifteen century, the lake was named Ho Hoan Kiem, based of a legend that is quite similar to King Arthur and the Lady of the Lake's legend.
While fighting against the Chinese, King Le Thai To has in his possession a very valuable sword. After 10 years of continuous struggle, the King finally defeated the Chinese and reclaimed Vietnam's independence. One day, while sailing on lake Luc Thuy, a large turtle appeared. The king drew his sword and pointed at the creature. The turtle immediately grab hold of the sword with its mouth and submerged. The king mourned the lost of such valuable sword, demanded that the lake be emptied and dredged. Both the turtle and the sword were not found. The king realizing that the gods must have lent him the sword to drive back the enemy, but now that Vietnam is free, the sword must be returned. King Le Thai To named the lake Ho Hoan Kiem or Lake of the Returned Sword.
Since the reign of king Le Trung Hung (XVI century), every king in the Le dynasty, and Lord Trinh have all contributed to the beautification of the lake. Lord Trinh Giang built Khanh Thuy shrine on Ngoc island on the north end of the lake. He also had the two man made hills built across from Ngoc son Shrine.
At the end of the Le Dynasty, Khanh Thuy was destroyed by Chieu Thong. A philanthropist named Tin Trai built Ngoc Son pagoda. Ngoc Son pagoda was renamed Ngoc Son shrine during the reign of Thieu Tri III (1843) because it was no longer a Buddhist shrine. Instead, Ngoc Son is a shrine to Van Xuong, a deity, in charge of literature and the various tests required to become a mandarin. It is also a shrine to general Tran Hung Dao, a national hero responsible for many victories against the Mongols.
Since then Ngoc Son has gone through many renovations, one of which was the addition of Thap But (Pen Tower) on the hill which was once called Dao Tai. Three words inscribed on the tower "Ta Thien Thanh" or "write on blue sky". Inside the gate a pool resembling the shape of an ink well was added. Beyond the ink well is The Huc bridge or "where the sun light is absorbed". The bridge leads to Dac Nguyet Lau or "Moon Light tower" - Ngoc Son shrine. Beyond the gates to the shrine, there are two walls called bang Rong and bang Ho (dragon and tiger slate) where the names of those who passed the national test are inscribed.
On the southwest end of the lake is Thap Rua. It was rumored that king Le Thanh Tong used to fish here. Lord Trinh also built the structure to house his entourage while visiting the lake.