What ancient construction technique has particularly fascinated you?

One of the building consolidation processes of the ancient period that I particularly like is the process of pouring lead into cavities drilled into the rock.

These are called "keystone" cuts and have been observed in ancient Egypt, and ancient Peru/Bolivia (with bronze, not lead). Because of these things existing on opposite ends of the world, many believe ancient Peru/Bolivia had contact with ancient Egypt or other ancient cultures. It is more likely however that different cultures from different continents have arrived at the same engineering solutions/conclusions without knowing each other simply because they had the same intuitions.