"If the Wall should ever fall, all the fires will go out."
Qhorin Halfhand to Jon Snow.

The Wall is a colossal artificial fortification made of ice which stretches for 100 leagues (300 miles) along the northern border of the Seven Kingdoms, and 700 feet tall, that defends the realm from the creatures and peoples lying beyond it. It was constructed by Brandon the Builder following the original war with the Others eight thousand years ago, using both magic and mundane means. The magic in the Wall is said to be so that only sworn brothers of the Night's Watch, the ancient military order founded to man and defend the Wall, can pass through the structure: however, this has largely passed down as myth, as have the very enemy that the structure had been built to keep out in the first place.