What allowed china to develop its civilization with little outside interference

The Great Wall.

Explanation:

The Great Wall of China is a defensive structure built between the 3rd century BC and the beginning of the 17th century to protect the ruling Chinese dynasties from foreign invaders.  The wall, which has an extension of more than 21,000 kilometers, is for the most part about eight meters high and 6-8 meters wide. It is built of earth, stone and brick.

The wall was born in the 200s BC by uniting separate walls to protect China from attacks by northern nomadic tribes, and its newest parts were built in the 17th century. At some point, the entire Chinese male population participated in the construction of the wall. The wall became redundant in 1644, when China extended north of the wall as well. The wall eroded for centuries until, in the 20th century, it was raised as an important national symbol in China.

well that depends on how deep the hole is. you'd first go through te crust of the earth, then the upper mantle, then the lower mantle then the outer core, then the inner cor then you'd be stuck in the core to melt.