What Do We Know About the Pyramids?

We know that the pyramids were built by gangs of workers who were summoned to work on the building site under a sort of national service or corvary system. We don't have ramps because they would be taken down when the pyramid was completed but we have other forms of evidence such as campsites where these workers lived and cemeteries for those buried there. The scale of the pyramids I suppose is one of the things that makes people doubt that people from the past could construct them. Well it's often estimated that Kufu's great pyramid has over two million blocks in it but actually we can't see directly inside it.