Tuerny said:
How useful is it to someone not building a campaign based on middle ages Europe?
There are lots of different elements you can use as building tools: feudal system, aristocracy, cities, guilds, generating buildings, agrarian society, religious hierarchies, magic and its practitioners, macro- and micro-economics, resource extraction, industry, services...
You can pick and choose what applies to your campaign. I strongly recommend you research in depth the culture you're basing it on!
-blarg
EDIT: For ancient Greece or India, a lot of those elements apply. Some things are different: in Greece there was less use of iron, wood was comparatively scarce, there was a slightly less convoluted aristocracy, and religious practises were obviously different. The rest is not so radically different from medieval Europe. They had cooler catapults.
In India, there are a few more elements that are different. Religion is severely fragmented - there were literally thousands of different religious practises. Kingdoms tended to be all over the place, and there were a lot of wars. Water was very important, rivers were often sacred, forests were more like jungles. I have no idea how magic would fit into all of that. Research, research, research!