Take some of this with a grain of salt, because magic's changed at home since I or my colleagues did some of this, and a bunch of it is application of things we've dug out of ruins. But we've done things like protecting cities from monsters with huge barriers to keep them out, or huge cannons, or ship's engines, or even personal protection. They're technological constructs, but with magical cores that give them direction. Some of those things need to be tuned from time to time, also, so you need to get in there and adjust the formula.
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