Hello and thank you for your interesting post.
If you would like to simulate the actual inclines of a specific route, you can use altitude and distance data from Google Earth. If you just want to simulate what it's like to climb on the Great Wall in the mountains, you will need to use a variety of uphill and downhill slopes varying from level ground to steep angles. In the desert, the terrain is relatively flat. In the mountains, it is far from it. There are places where the angle approaches vertical – 80 degrees perhaps – but people don't usually climb these areas as they are quite dangerous, and obviously treadmills can't accommodate this. The slopes change constantly and the height and width of the steps also vary widely. Some steps are more than knee high and others are just inches. Some steps are wide enough for two or three strides and others are too small for your foot. And the steps are not necessarily horizontal. So in summary, you will need to simulate a great variety of constantly changing terrain, and in my opinion it won't be easy to realistically simulate with either a treadmill or a stepper. But I encourage you to try and keep us informed of your results!
Oh, and for a realistic simulation, you will need to carry a heavy pack and subject yourself to weather, hunger, thirst, slippery and unsure footing, and several kinds of mental stress.
Here are some examples of how steps vary.

These steps are recently restored.

These aren't.
Best wishes to you.