They had 2 huge obelisks on different spots seperated many KM form eachother and they discovered that arround the same time of day the shadows on those obelisks where different.
And there for the world should be round/roundly.
Sorry dude but, you're not even wrong; those were Greeks. The ancient Egyptians i.e. the ones who built the pyramids, knew the Earth was flat.
https://www.youtube.com/watch?v=8On7yCU1EjQ <-- This is wrong you get the same results if the Sun is small, close and orbiting above a flat plane.
"Eratosthenes" <-- Errortosthenes.
Your obelisk story is a modern fabrication crated by baal earthers; there's no hieroglyphs depicting this. The ancient Egyptians believed the Earth was flat and motionless; they depicted this fact in their literature and architecture.