This was done deliberately by Bran the Builder to strengthen the structure.
However, in many areas the lower third or so of the Wall is actually built into naturally-occurring hills. The Wall's height is not uniform, and in some areas comes close to 900 feet in height. According to myth, spells are woven into the Wall to prevent it collapsing from mundane causes and to keep certain entities, such as the Others, from passing. At its top it is wide enough for a dozen horsemen to ride abreast, and is thicker at the base. The Wall stretches for 300 miles from the Gorge to the Bay of Seals and is approximately 700 feet tall. The Wall is made of solid ice, althouth the top is compacted with gravel near the inhabited castles.
