I'm always amazed at how ancient people were able to build these gigantic structures without any modern tools. I saw a picture of a Babylonian ziggurat once, and I'm telling you, I would have no idea where to start to make something so gigantic.
From what I understand though, these projects took many decades to complete. It's not like today where you start on a building and it's finished in under a year. So while it is amazing ancient people were able to build these, I don't think it's totally outside the realm of possibility!