The Walls of China

 1.长城

Date of construction: 6th century B.C. through 16th century
Built to keep out: Invaders from the north
Status: Tourist attraction and UNESCO World Heritage Site

2.三北防护林（修造绿色万里长城活动）

Date of construction: 2002 through ~2050
Built to keep out: The Gobi Desert
Status: Mixed

3.中国国家防火墙（GFW）

Date of construction: 2002 (and even earlier) to present
Built to keep out: Ideas
Status: Trivial to circumvent but still annoying