Yeah, Brit and Gaul indeed have a higher priority.
I'm not too sure about that artist impression, though. The tomb I'm talking about is this:
There is a decent description at its livius.org article, the dimensions match those at the French Wikipedia page, and more photographs at the Wikimedia Commons entry.
There exists `art/actors/units/global/`; I'd suggest a similar folder for structures, not just for wonders belonging to none of our civilizations (e.g. Stonehenge, the Pyramids, the Hanging Gardens), but also for palisades and other such shared, generic structures.