I'd need some more information before I'd give advice.
You said that there would be some magic, but I'm unsure of these aspects:
1) Is there going to be a feywilde/avalon or shadowfell/purgatory/sheol/hades? Is there a hell? Will you be following Christian Cosmology, Celtic Cosmology, or is it going to be a strictly physical universe?
2) Is magic rare, weak or hidden?
If it is rare, I'd probably require certain objects or talismans to both include and limit the magic. For example, a nail from Jesus' crucifixion welded to a sword's hilt might make it a holy avenger. A relic (bone, mummified flesh, object owned) from a saint might grant a single clerical power.
If it is weak, then I would probably tie fluctuations of magic to specific astrological cycles or demand high payments (such as human sacrifice (arcane or shadow) or extreme asceticism(divine). In other words, I would make magic unreliable except under certain conditions.
If magic is hidden, then I imagine it would be more like D20 modern or the Buffyverse, only set in the 11th century. Here the feywilde or the shadowfell would be sources of monsters, or sources of power. The general disapproval of witchcraft by both secular and religious authorities would help keep magic hidden, though of course there was no active inquistion directed against the practice of witchcraft. This was the 11th century after all, not the 16th. I imagine though that proven unrepentant worshippers of the devil would probably have been executed nonetheless.
3) Are there monsters? I guess I'm wondering if you want a political game or a mythological one. If you are largely interested in feudal warfare or going on crusade, then you'll probably have to worry more about collecting followers, maintaining a stronghold, or mass combat rules.
If you're planning to have a dungeon crawling game, then you will need monsters. Nobody is going to want to fight humans over and over again unless they are taking over the place, and if they're taking over the place they'd rather intrigue and negotiate themselves into power instead of using brute force.
There are many ways to bring monsters into the game while still maintaining a sense of historical Europe. Dragons were associated with greed or the devil, so a rapacious beast terrorizing the wilderlands or a wingless beast sitting on a lost horde in the deepest cavern in Wales. All you need for goblins is a Celtic barrow leading to the feywilde and some red caps. Undead can fit in as a defiled graveyard, a necromancer at the Cathedral School, or an old Roman vampire (I'd recommend
Allectus - Wikipedia, the free encyclopedia since he was killed by the army of the father of the first Christian Emperor Constantine).
You can't do too many monsters of course, or it will break the mood of a historical Europe. If you want to go whole hog on the whole "monster/witch hunting" in the 11th century, I'd recommend having one character be an exorcist in the church who is tasked by a bishop or abbot to figure out what is behind weird events. I'd suggest slipping in a few hoaxes.