Gungnae Fortress, also known as Guonei City, was the second capital city of the ancient Korean Kingdom of Goguryeo. This place is present-day Ji'an (集安), and the perimeter of its outer fortress measures 2,686m.
It was chosen to become the capital city by the kingdom's second ruler, King Yuri during the 10th month of the year 3 AD. The city was sacked several times until the rise of the 19th ruler king Gwanggaeto the Great, who greatly expanded Goguryeo's territory and made it a formidable power in the northeast Asia. When King Gwanggaeto died in 413, his son, King Jangsu, inherited the throne and moved the capital down to Pyongyang in 427.
Just before the fall of Goguryeo, Gungnae-seong fell to the Silla-Tang alliance when General Yeon Namsaeng, son of Yeon Gaesomun, surrendered the city in 666 AD. The kingdom of Goguryeo itself fell in 668 AD when Tang army captured Pyongyang and took King Bojang and Yeon Namgeon into custody.