Walls of Halruaa

The Walls were a crescent-shaped mountain range that towered up to 20,000 feet (6,100 meters) over Halruaa.

Geography
Prior to the Spellplague the mountains formed a natural defense for the realm. Unfortunately, the range's height made it difficult for Halruaans to escape the effects of the Spellplague. Three passes connected the remnants of Halruaa to the rest of Faerûn.