The Chinese built a wall on top of mountains and they had less technology I just wonder why people are saying is difficult to built the wall?

considering it took 2000 years and while The costs vary greatly but it's safe to say the per mile cost is somewhere between 1 - 5 million dollars / mile. That applied to the entire length of the Chinese Wall: 13,170 miles x 1 million dollars = $13 billion to $65 billion. 300k to 500k workers and that was in 400-500 bc. estimates are it would conservatively cost 100 bil today.

So you are wrong it was not easy or cheap then. and that was using slave labor.