The core concept is a bastardized fusion of utility fogs, and the magical barriers of witches from the anime Puella Magi Madoka Magica.
Utility fogs are swarms of 100 micrometer robots, foglets, that have several arms, which they can extend and retract and "grab" the arms of other foglets.
Witches and their barriers are usually depicted as their own realm, with "familiars" roaming the place. Though neither witches nor familiars are immune to conventional weaponry, some can still tank a lot.
Barrier Magic gets it's name from the "barrier" of utility fog, that separates the magic user from the outside world, similar to what we see in Madoka, though it can't warp space and doesn't necessarily look like an LSD trip. A barrier can easily get very huge, and utility fogs are supposed to be able to manifest just about any building, so that's a similarity.
Magic always happens in barriers, but it's possible for a magic user to detach slivers of their own barrier and remotely control it. These slivers are called familiars.
Fogs "move" by forming layers that slide on each other. Basically, the top layer moves from foglet to foglet, pulling itself to the next. Once the top layer moved far enough from the edge of the bottom layer, the bottom catches up.
Barriers get most of their energy in the form of radiation. While narrowcast is less-than-ideal in most circumstances, barriers can get very large, so they're both easier to hit and are fine with a decreased energy density, a match Made In Heaven.
Now, there are magic users, mahoutsukai as I call them 'cause once you get Dorohedoro, you can't get out. Mahoutsukai are supposed to be so destructive, they can raze an entire city when going all-out.
That's okay, because they're all gone now, but it still leaves the question of how can utility fogs achieve such destruction without the barrier utterly annihilating itself in the process?