Verderer
I don't envy you the task of building a historically accurate campaign world, in fact I would normally advise against it. However, it is fascinating and frankly the times that I have attempted to work on such a project have lead me down into a mental bog obsessing over details. You may want to set a “Point of Divergence” historically far enough back that you can say; “From this point on all bets are off!” and allow artistic license to allow the flexibility to deal with the inevitable situations where someone will say “It did not happen that way!”, “That's not historically accurate.” or worse yet “God, does not work that way!”
Like the mechanics of the game, allow history to be a guide, and pardon the reference, not be “cannon”. If you want your players to enjoy the game, otherwise you might as well be teaching a class on medieval history, concentrating the theology of the crusades. “Yes there will be a test, with extra credit if you come in period costume.”
Make sure your players are aware of what you are trying to do for their entertainment, not argument. I think history and religion are fraught full of dangers, heck we are still fighting REAL wars over this stuff. I don't know your group of players but in general I think those who would really be interested in such a game will be intelligent and well educated in medieval history. I mean role play gaming did not earn a reputation as a “nerd” culture for nothing. They, the players, may end up being a big source of frustration, or they can play the most interesting characters.
No matter how bad or good you think some movies are there may be some to consider. As mentioned before “The Name of the Rose”, “El Cid”, “Kingdom of Heaven”, “The Messenger”, “Ladyhawk”, and the latest “King Aurthur” even “Robin Hood: Prince of Thieves” likely all have elements you may want to include, or perhaps have your players watch for inspiration. Also I think the BBC “Cadfael” series if you can get a hold of it may hold your interest.