well, just to let u know that as a mage that can sneak, u can cast your spells while u sneak (so long as u're not too close to the enemies) & still the enemies won't notice u. of course all is dark but if u hover your cursor around, u'll bound to 'find' the enemies when u see their HP bar pops up.
hope that helps. & i think this is a bug in the demo. don't know if this trick still work in the full game.