 Although the number of towers was later scaled back to 1200, the project, which started in 1569 and lasted two years, marked the first large-scale use of hollow watchtowers on the Wall.Up until this point, most previous towers along the Great Wall had been solid, with a small hut on top for a sentry to take shelter from the elements and Mongol arrows.In contrast, the Ji–Chang towers built from 1569 onwards were hollow brick structures, allowing soldiers interior space to live, store food and water, stockpile weapons, and take shelter from Mongol arrows.