Thomas also tells Anton that the seventh level of the Twilight is the Others' paradise, where they can exist in peace together (upon death, Others just vanish into the Twilight). Merlin has hidden the Crown of All Things in the seventh level of the Twilight.
The seventh level of the Twilight is considered the Others' sanctuary, an area where they can live in harmony together (upon their demise, the Others vanish into the Twilight). Merlin has hidden the Crown of All Things in this particular realm.