So first of all, have a good look at these pictures:
See how exact the stones in this ancient Inca wall in Peru are placed together? They say that you can't even put a sharp razor blade between them! And that's without mortar or anything to bond them together! Imagine how difficult it would be to get a hard block of stone that even, level and smooth, let alone the right angle to fit perfectly alongside another block!
And what about this:
They could also do perfectly smooth curves! Not just in corners of rooms, but when sculpting those giant stone statues!
Some modern-day engineers have had a closer look at the remarkable workmanship of Ancient Egypt and other impressive ancient ruins, and have concluded that getting the curves, angles and symmetry so exactly right is almost impossible, even with today's technology and know-how!
One of the books detailing these things is Lost Technologies of Ancient Egypt: Advanced Engineering in the Temples of the Pharaohs by Christopher Dunn. Reading it will convince you that Ancient Egyptians had knowledge and skills above and beyond ours today!
When I discovered these fascinating things, I knew I had to incorporate it into my writing - and in fact, this advanced technology of the ancient world is a fundamental theme of my Rhuna Fantasy book series, and since it's Fantasy, I'm allowed to theorize all I want as to how, why, who and where (but it's not aliens!).