The grounds of the Imperial City are protected by fortified ramparts 2 kilometers by 2 kilometers, and ringed by a moat. The water in the moat is routed from the Perfume River through a series of sluice gates. This enclosure is the citadel (Kinh thành).
Inside the citadel is the Imperial City (Hoàng thành), with a perimeter wall some 2.5 kilometers in length.
Within the Imperial City is the Purple Forbidden City (Tử cấm thành), a term similar to the Forbidden City in Beijing. Access to the innermost enclosure was restricted to the imperial family.
According to VietKings (Kyluc.vn)