The walls formed by a great crystalline globe that enclosed the flat world of Arda, separating and protecting it from the surrounding Outer Void. Also called the Walls of Night, or by the ancient Elvish name Ilumrambar, they extended upward above the world and downward beneath its roots.

The Walls were impenetrably hard, but they contained at least one portal into the Void beyond: the Door of Night. According to some accounts this was created by the Valar to allow the passage of the Sun (with the equivalent Gates of Morning in the east). The Door of Night was also used to expel Morgoth from the world at the close of the First Age, after which it was perpetually watched by the Valar to prevent his return.²

The idea of the Walls of the World evolved over the years in Tolkien's imagination. In earlier versions of the stories, they did not form a solid sphere, but simply surrounded the world as walls might surround a city. In this conception, the walls had a top and could be crossed, and remnants of this idea survive even into the published Silmarillion. So we have references such as '...[Melkor] passed therefore over the Walls of Night with his host...' (Quenta Silmarillion 1, Of the Beginning of Days) something that should have been impossible in the later conception.

Christopher Tolkien addresses these issues in volume X of The History of Middle-earth. Though no complete reconciliation is realistically possible, he does make the useful suggestion there that we can imagine two sets of Walls. On this model, outer walls (Éarambar) held out the Void around Arda, and can be seen as a spherical 'shell', while inner walls (Ilumrambar) surrounded the world more like an encircling 'fence'. While not a complete solution, this does go some way to resolving the apparent contradictions.