There are no specific mechanics for scribing circles. If you know how to do one (because you're a spellcaster or one told you how to do it) you can just do it. I mean, it just stops pretty much anything not on The Heavyweight List. Or at least anything wholly magical/summoned. Trouble is, though, you're still at the mercy of mortals with film cannisters or guns, and the things can just wait till you get tired or break the circle yourself, so it's not much of a refuge.