I think the gravity light bending effect can work. You don't have to actually create a gravity well like that, you just have to make the light behave like it is in a gravity well. It might seem like cheating, but I would allow something like this, go for the effect you want without the solar system crushing sideeffect.
BUT in this case, I wouldn't allow it. We are talking 13th century here, and the magic should be as rough as the people's understanding of the world. Newton doesn't enter the stage for another 300 years, and aristotelic concepts are still well in place. Don't even get me started on Maxwell (magnetism).
So the magic would probably be much more crude and direct as well. Moving earth around, mainly. You could maybe get magnetism into it as "the love of iron", as Pratchett put it. It pulls iron, but nobody really knows how or why. But even that feels like a stretch.
As far as concepts go, Earth was always associated with standstill, and since Aristotle interpreted motion as any change, not only movement, earth could be used to stop things from doing anything. Stop a shield from bending, stop a log from burning, stop a man from moving, and so forth. You might want to look into this a bit further for inspiration.
When it comes to maneuvers, don't dwell too much on the spell itself, think about what it's supposed to do and then think about how a spell might accomplish that. If you want somebody to be "tripped up", a minor localized earthquake could do the trick. If you want someone to slow down in order to escape, send a wave of standstill towards them. If you want to keep a "firm stance", remove any motion from your legs, so neither you nor anybody else will be able to move them around.
