Great Wall of China
The earliest version of the Great Wall was built 400 years before the birth of Christ. Even the more complex version we see today - really a series of walls that were built and rebuilt over centuries - is between 700 and 500 years old. So without diesel engines or computers, the Chinese built a solid defensive wall that winds its way across 4,000 miles of forbidding, often mountainous, terrain. And it even looks good. That's some feat.