http://www.totalwar.com/community/medieval1.htm
With the Vikings Invasion expansion pack, you get a fuckload of new military units, and can now use boiling oil in siege defense. Also, you can easily mod a few text files to make the game as hard as you like. YOu can wage war with Catholics, Orthodox Christians, Pagans, Muslims - heck, you can even drop an assasin on the Pope to see if that can do something about that little Excommunication problem....