Walls of Halruaa

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

Geography
Prior to the Spellplague the mountains formed a natural defense for the realm. The three sections of the mountain range–the Nathaghal, Lhairghal, and Muaraghal Mountains–formed a stout border.

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. The Azhal Pass and Talath Pass were difficult for a large group to travel. Only the wider High Aluar pass offered a secure route for any sizable group.