Agriculture, by far. As someone who knows almost nothing about it but has tried gardening, figuring out agriculture from scratch seems impossible.

I remember watching a you tube video about how the cement used in ancient Roman architecture has proven to be super durable. I don’t remember details but it was interesting.

The Chinese repeating crossbow always impressed me, very high rate of fire. Not as much penetration as medieval crossbows but could be enhanced with poison tip arrows.

I always thought the yakhchal was pretty amazing. Ice house that can make ice.