Now That We're on Expansion 2...

I'm a little surprised we're 2 expansions in without this addition:

Clearly mark the technology on the tech tree that unlocks each civ's special unit or building, or at just make the icons for the specials include a border color/symbol/animation that tells you where it is. This is such a basic thing but it would make such a huge difference. (I see in GNK they finally added descriptions of these objects to the main menu screen, only the on-roll over hint text only displays AFTER you've started the game.)

BTW I'm looking for the Aztec special building right now. Where is it?

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World Icon
Aztec

There are the floating gardens, click on it.

It's right under your nose, replaces The Water Mill, one of the first few techs you will gain.

This has been in the game since day 1, just use the civilopedia and look up the building and it tells you which tech unlocks it.