Outside the walls a series of semicircular defense lines were constructed in the path of the Japanese advance, most notably an outer one about sixteen kilometers (10 miles) from the city and an inner one directly outside the city known as the Fukuo Line, or multiple positions line. The Fukuo Line, a sprawling network of trenches, moats, barbed wire, mine fields, gun emplacements, and pillboxes, was to be the final defense line outside Nanking's city walls. There were also two key high points of land on the Fukuo Line, the peaks of Zijinshan to the northeast and the plateau of Yuhuatai to the south, where fortification was especially dense. In order to deny the Japanese invaders any shelter or supplies in this area, Tang adopted a strategy of scorched earth on December 7, ordering all homes and structures in the path of the Japanese within one to two kilometers (1.2 miles) of the city to be incinerated, as well as all homes and structures near roadways within sixteen kilometers (10 miles) of the city.