Pathing clearance is determined in the UnitMotion component, I believe, with PassabilityClass. There are a couple of standard sizes. The bigger the passability class the more problematic the pathing performance. 

It would be so cool if the Minoan priestess could wave snakes around. 

Maybe both, but the Tachara is the Achaemenids' major "fancy" special building.

Would it be too hard to make it so if units get hit from the back it does more damage? I am thinking for strategy purposes, then there would be an advantage to attacking from behind

I kinda like how the existing dirt decals deal with the transition at the edge. Less straight makes it look more natural imo: Apart from that, the pebbles and the grass are nice. Maybe make the grass a little lighter, so it doesn't stand out too much and it fits with the colors of a mediterranean biome.