The Ruined City of the Giants, also called the City Ruinous, was a ruined city in the Wild Lands of the North where giants once lived, during the Age of Conquest, possibly before Harfang was built. It was built during the Age of Conquest, and fell during the Dark Age. Jill Pole, Eustace Scrubb and Puddleglum fled from the giants of Harfang to the Ruined City.
The city was located far north from Narnia, and near the castle of Harfang, which was also close to the northern border of the Wild Lands of the North, and presumably once the capital of it.
Although the city's original name is unknown, it was once the centre of a vast empire, according to a eulogy written to one of its Kings:
"Though under earth and throneless now I be, Yet, while I lived, all earth was under me."
Eventually, the city was gradually reduced to ruins, and at some point the giants of Harfang built their castle to the north of the ruined city (presumably before it fell). Finally, all that remained of the inscription was the final two words: "UNDER ME."
Following the third Sign, given by Aslan, Eustace, Jill and Puddleglum went through a small doorway at the foot of the plateau, on which it was built, and entered Underland.
It is possible that Jadis may have been the one responsible for the construction of the city. After she first came to Narnia, she was banished to the north, where she remained for 900 years, and she did use Giants for her army. With that in mind, it's possible she may have built herself a small kingdom in the Wild Lands of the North until she eventually returned to and conquered Narnia. After she left the Giants, without her rule or guidance, they basically degenerated into their more brutish selves, and their kingdom gradually fell into ruin around them.