The Dawn Patrol is an AWESOME World War I movie about British fighter pilots. Don't let its age fool you -- it's even most awesome at the end.
http://www.imdb.com/title/tt0030044/
"Lawrence of Arabia" is one of the most epic films you could possibly see. Gorgeous cinematography, huge armies clashing on the desert, one of the most famous film scores of all time, and Omar Sharif. And Peter O'Toole. It's also set during WWI, but since it mostly concerns hordes of Bedouin warriors, there's a strong medieval Eastern flare to it.
http://www.imdb.com/title/tt0056172/
And the TCM trailer, also awesome: http://www.youtube.com/watch?v=bSlq1CNQwEc&feature=related
A similar one is "Khartoum," also involving a British attache charged with organizing the defense of the titular Egyptian city against the forces of the Mahdi. Also historically-based, with epic desert armies clashing, and this time starring Charlton Heston. A good flick.
http://www.imdb.com/title/tt0060588/
The thing about asking for uncommon epic medieval war movies is that in order to have tons of epic action, they have to be big budget, which in turns means they probably were mainstream and got a lot of attention. By "uncommon" I take it you mean "little known today."
So let's see. There are various movie versions of Joan of Arc, some of which are probably good.
Kenneth Brannagh's take on Shakespeare's "Henry V" was supposed to be excellent. It's definitely a medieval war movie. http://www.imdb.com/title/tt0097499/
There's also one with Charlton Heston played El Cid, the medieval Spanish mercenary-hero. Heston is always fun to watch.
http://www.imdb.com/title/tt0054847/
I've also heard "The Thirteenth Warrior" recommended, but haven't seen it myself. Antonio Banderas.
http://www.imdb.com/title/tt0120657/