[thejeff]

Late era seems to have less amazons, druids and shamans.

Era's probably the largest source of difference.
Terrain will have an affect too.

Higher indy levels will get you more defenders with mages, even when there aren't recruitables there.

Speaking of the Blood Druids:
Does anyone else wish you'd get them instead of regular ones when you have to fight them? Is there a site you can get them from?