Erebor
The Lonely Mountain, known in Sindarin as Erebor, referred to both a mountain in northern Rhovanion and the subterranean Dwarven city contained within it. In the latter half of the Third Age, it became the greatest city in Middle-earth.
Erebor. Built deep within the mountain itself, the beauty of this fortress city was legend. Its wealth lay in the earth, in precious gems hewn from rock, and in great seams of gold running like rivers through stone.