The Wall is a 700-foot all, 300-foot thick structure that spans 300 miles along Westeros’ northern border, from the Bay of Seals in the east to the Gorge in the west. Legend has it the was built from ice, stone, and earth (and some magic, but we’ll get into that), 8000 years before the events of A Song of Ice and Fire, erected by King Brandon Stark, a.k.a. Bran the Builder, to protect the Seven Kingdoms against White Walkers and other creatures of the Great Other. 

“Almost seven hundred feet high it stood, three times the height of the tallest tower in the stronghold it sheltered. His uncle said the top was wide enough for a dozen armored knights to ride abreast,” we learn from Jon Snow’s perspective in A Game of Thrones. “The gaunt outlines of huge catapults and monstrous wooden cranes stood sentry up there, like the skeletons of great birds, and among them walked men in black as small as ants.”

This doesn’t have anything to do with the show’s mythology, but George R.R. Martin says the idea for the Wall came from a visit to the historical site of Hadrian’s Wall in Carlisle, England.

In a 2014 interview with Rolling Stone, Martin explained,

“I can trace back the inspiration for that to 1981. I was in England visiting a friend, and as we approached the border of England and Scotland, we stopped to see Hadrian’s Wall. I stood up there and I tried to imagine what it was like to be a Roman legionary, standing on this wall, looking at these distant hills. It was a very profound feeling. For the Romans at that time, this was the end of civilization; it was the end of the world. We know that there were Scots beyond the hills, but they didn’t know that. It could have been any kind of monster. It was the sense of this barrier against dark forces – it planted something in me.”

Hadrian’s Wall is about 73 miles long with varying width and height up to about 10 feet wide and 20 feet tall.