Long before the Chinese started constructing artificial islands, they built 7 artificial hills/fortresses in NE China, hoping to slow down an impending Soviet armor thrust into Beijing. The Soviet cold war goal was to capture Beijing, install a puppet government and divide China into half; knowing full well the Soviet Far East did not have the manpower to conquer entire China while facing a much more powerful NATO in due course.
From 1969 to 1978, the buildup of Soviet troops along the Chinese-Soviet boarder accounted for 80 percent of the total increase in Soviet military manpower. In 1973, Soviet ground troops long the Soviet-Chinese frontier were increased from 23 divisions in 1968 to 45 divisions -- including 8 tank divisions. (Page 34, from Adversaries to Partners?: Chinese and Russian Military Cooperation after the Cold War, By Ming-Yen Tsai)
Here are pictures of fortresses number 4 located in Ejin Banner of Innter Mongolia: Totaling 80 acres in size, 340 meters in diameter and a radius of 160 meters. Elevation of 51 meters and the fortification is surrounded by a gully of six meters deep. Construction started in March 6, 1963 and completed by the end of 1969. Total garrison was 4748 troops. The purpose of those artificial artificial hills/fortresses was to slow down the Soviet blitzkrieg as they were not expected more against such a mighty foe.
Another one from Zhengding County, Hebei Province.