There are really people who think Native Americans couldn't have built the Cahokia Mounds? I mean, they're big piles of dirt. Just takes a lot of dirt, time, and manpower--not any really special technology.
Likewise, the Pyramids were also quite doable with the available technology, it just took a lot
of time and effort.
I think that's the part modern people (maybe just Westerners) have a hard time grasping: construction projects lasting for decades or even centuries, generations of workers participating. The idea of sticking with anything that long must seem a bit, well, alien.