Use some real things, so that the player can know the why and how of certain time period.

If you describe extensively the events they can be completely fantasy.
I use some ''real world'' events in my world, but slightly modified.

There was crusades but they were ordered by the gods themselves to defeat the others gods through the extermination of all the followers.
(In my campaign, a god powers depends on the faith put in him by it''s followers [people who beleive in him])

This is just an example of things you can reuse.
In reality crusades were a way to get tons of gold.

-* So many things to do, so little time to spend. *-

Just my opinion, but I think a game should always have at least half original elements. History is a good guide to what kinds of cultures can realistically evolve, but it''s like when you write a poem, you''re supposed to cut out the unnecessary words so you get something more intense than real life. Using a completely real historical setting may be educational, but it probably won''t be a very rich and vivid world because the signal to noise ratio is pretty low.