A role-playing game topic I see regularly is the effect magic’s existence has/should have on historical architecture and, more specifically, their defenses against various threats. Namely, magic’s existence rendering those defenses irrelevant. Some essays include how game masters should rethink/redesign castles, treasure vaults, prisons, et. al.
To me, the solution is obvious: add a nullification of all magic effects to the mundane defenses of any object or location. In short, anti-magic. For this to work, anti-magic needs to be available proportionate to the level of magic in a campaign world. Thus so, historical defenses require no rethinking at all.
The simplest answer is most likely the correct one.
Personal magical defense will be the subject of a later post.
ADDENDUM: While I do see a viable market for spell-casters to specialize in creating anti-magic wards for third parties, non-casters would—rightly—be concerned about the potential conflict of interest in the hired mage. Another option that does not require one mage to stop other mages would be preferable.
I recently came upon a blogpost about an example of just this option: Voidstone and Voidsteel on Thalian Musings.