That's because it first chooses the placement, without knowing whether the map supports it.

Beautiful! You're doing a lot better with UV mapping. Just try to minimize texture stretching and maintain texels 

There are several ways to make Hamippoi work. I opted to make them all 1 actor and unit. Other ways could be:   Cavalry can train cheap melee footmen called "Hamippos." Either as a default feature or unlocked with a tech.  A tech called "Hamippoi" which boosts cavalry. A "Hamippos" infantry unit with a strong aura benefiting cavalry.  Mix and match any of the above. I went with the one unit option for this release. Next release I could try something else. 

In the pull request, they are simply called "Achaemenids" because of the current 1-word nomenclature for civs in the base game. I would like to change this where applicable (specifically when using dynasties for distinction as is the case for Achaemenids), but that's a different Pull Request and requires some discussion. 

Thats pretty cool!