My words literally betray me.
I was wondering if technologies had to be built in order (from left-to-right) in each of the respective rows. I made initially made that assumption, based on how a lot of other games work, but after re-reading the rules, there's nothing that says you have to.
If you can build them out of order, I question whether certain elements of the game are O/P. For example, The Great Library potentially giving you a level 5 tech for 4/3 gold in the First Era.
each row must be built in order.