 These positions, however, were not for a linear defence but rather a regional one in which walls did not feature heavily, and offensive tactics remained the overarching policy at the time.Hongwu's son, the Yongle Emperor (r.140224), continued his father's policy of active campaigning against the Mongols, and in 1421 moved the Ming capital from Nanjing in the south to Beijing in the north, partially to better manage the Mongol situation.Construction of walls in stone and earth began under Yongle's reign in strategic passes, when signal towers and ditch systems were also established.