Zhengtong Emperor
In 1449 Esen Tayisi led an Oirat Mongol invasion of northern China which culminated in the capture of the Zhengtong Emperor at Tumu. Since then, the Ming became on the defensive on the northern frontier, which led to the Ming Great Wall being built. Most of what remains of the Great Wall of China today was either built or repaired by the Ming. The brick and granite work was enlarged, the watchtowers were redesigned, and cannons were placed along its length.