Quan Chuong Gate, the only ancient gate that remains in Hanoi, was built in 1749 during the Le Dynasty. It’s located in the eastern part of the Old Quarter and has suffered from the ravages of time, neglect, war, and urban expansion that destroyed the other 15 original city gates.
Looks like a little fort - It's the Old East Gate (Cua O Quan Chuong) erected near to the Bach Ma Temple. It is well preserved and looked very historic. Read how the Old East Gate comes about in my "Things to Do" Travel Tip under "Bach Ma Temple"