Wow! I didn't realise it would raise such a discussion!
So, here is the answer: I am, in fact, standing on a wall erected by another civilisation entirely; It's the Great Wall of
China!
This section, "HuangHua" (=Yellow Flower, 60 km outside of Beijing), is almost entirely untouched since it was first build during the Ming Dynasty (1368-1644). If you get a chance to visit the Great Wall, I really recommend this site. It is quite "un-touristified", and my party were basically the only people there. If you are a
bit more adventurous than I am, there are better preserved sections of the wall at HuangHua, but getting to them requires hiking gear and you go there at your own risk!
The square blocks are apparently lime
stone, and a cement mix using rice and
egg whites was used. Most of the center of the wall was just filled with rubble (dirt, stones etc).
There are, of course, other portions of the wall that have been
restored to look very much like when it was originally build (such as Badaling and Simitai), but there tourists flock in huge numbers.
Contrary to popular belief, it is not visible from the Moon (and claims to that effect are currently being removed from
Chinese school books).
//Auto