[Anaraxes]

Is it harder and more expensive to build a pentagram wall than the regular sort? If not, there's no reason not to do so, and they'll be employed routinely (at least as widely as the knowledge of the technique allows). Not just castle walls, but any building would have them, the way all (modern) real-world buildings have grounded electrical systems.

If that's not the desired world-building goal, then you'll need a way to limit the proliferation of pentagrams. One traditional way is the trope that mages are scarce and busy, but if just any architect or builder can do so, without needing any special inborn talent or arduous training, then you either get a lot of cheap pentagrams, or else have to invent some other barriers to their deployment.