[G 12 / Mongol] attacks from the northeast had indicated defence weaknesses and in 1551 work began on fortifications east of Beijing, which itself received a new wall in 1553. To the west of the capital, building and repairs to existing walls took place as well. Proposals to repair existing towers as well as build new ones to the south were made in 1556. And in 1557 and 1558 more repair work was carried out in Shaanxi and on the Datong wall, among others. Extensive fortification work was conducted from 1567 to 1573 and on into the 17th century. In 1565 Shanhaiguan was strengthened. The most important work was, however, considered to have been on the northeastern frontier.
