They based it off of El Caracol, located in Chichen Itza. The Yucatán landscape was flat so it was used to observe the night sky with no obstructions around. They needed an adjacency bonus and farms and plantations are built on flat land early game.That's true... but then, why no Mountain adjacency? I mean, an observatory would perfectly have a better "astronomical" output if the astronomers would just have to go on a 2-day trip to go a little higher to see the stars...
Plus, I don't see why plantations would then, following this logic, allow astronomers to be more potents.
Except if you consider plantations & farms as open land with no light pollution, making it better to study the sky? But then, why do observatories would have better adjacency from districts that produce light pollution?
https://en.wikipedia.org/wiki/El_Caracol,_Chichen_Itza