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.

I’m dumb so, no particular technology comes to my mind. However, isn’t all ancient technology amazing? How our forefathers invented this and that, we’ve reached this place because of their efforts. That just makes any piece of human-made technology amazing, maybe except Nuclear Bombs. Although, I do think it must’ve been a proud moment for our forefathers when they learned how to make fires.

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.