Yeah, when you add magic, it doesn't really matter just what the object is. Systems of detections and negation come to mind, but that also brings to mind all those hundreds of arguments (that I've read; in any fantasy game universe) about how magic shouldn't be just another kind of technology, and that making it so drains out everything that makes it special. The last thread I worked on with it resounded with the words "Let magic be magic!" Heh.
Anyway, when enchantment comes along, there would probably be a method of negation the guards would run over the entire city. Something that would smother any greater spell; things of "Planar Gate" and "Finger of Death" strength. Although, considering magic can be used to make constructive activities easier, or possible in the first place, there would probably be a balance between what will keep the city safe and what the populace will put up with. This balance could shift around every few years, probably resulting in violence at the peaks and pits. There would also be the looming danger that stems from the fact that a very dangerous spell might be very small (the spell of Blend Spinal Contents). Or, that it's like flash stones and the blanket enchantment doesn't get a chance to stop them as they go off.
