The Temple of King Erzheng is a small temple to commemorate two heroic women who fought against China in Vietnamese history. It is also one of the few temples built in Hanoi, Vietnam to commemorate women. The shrine is currently under repair. If you want to go, you should wait until it is repaired. The time of repair is not clear yet. There is a small lake in front of the temple, but unfortunately the lake water is quite average.
The temple enshrines the two sisters in Vietnamese mythology, two female generals who "resisted" the "national oppression" of the Eastern Han Dynasty, and the Vietnamese national heroines killed by Chinese invaders.
This temple located south of Hoan Kiem Lake just has some small but gorgeous stone carvings in front of the hotel. The door is closed on Sundays. I think it is the locals. If you have a look in this area don't make this special trip.
The Temple of King Erzheng enshrines two women. They are two anti-oppression female generals in Vietnamese mythology. This is also relatively rare in the world. Generally local people will go there to pay respects every year and festival.
The history of Vietnam is neither long nor short. The history of Hanoi is almost the history of Vietnam.