Walls of the World
- "They are as ice and glass and steel, being above all imagination of the Children of Earth cold, transparent, and hard. They cannot be seen, nor can they be passed, save by the Door of Night."
- - Rúmil of Tirion, The Ambarkanta
The Walls of the World were the walls that surrounded Arda and marked its boundaries before it was 'bent' at the time of the Downfall of Númenor.
J.R.R. Tolkien, Christopher Tolkien (ed.), The Shaping of Middle-earth, "V. The Ambarkanta: Of the Fashion of the World" p. 235