We know the pyramids were not built by slave labor. There were several months each year when the farmers had nothing to do, so they hired free farmers to build the pyramids. They were typically paid in beer and garlic, and we have found graffiti which basically amounts to different work crews smack talking each other about how slow they were. However, just about all other buildings in ancient Egypt were built with slave labor. They captured and kept tons of slaves for labor they didn't want to do, like pretty much every other civilization at the time.