Circles of protection, often a ring of salt, occasionally a set of runes, are a staple of supernatural fiction; a mystic shield that protects the protagonists and imprisons antagonists.

The problem is, a circle is a flat plane and we exist in more dimensions, so in order to protect us, the circle has to have more dimensions. But how are those dimensions defined and refined?

One version would be that the circle is actually a sphere and the width determines the height, though humans tend to be taller than they are wide, and with that math many circles would leave heads exposed. Something that is rarely explored, but could be a good piece to a story.

An obvious alternative is that the amount of energy used when empowering the circle determines how tall it is, and by habit most people make it taller than they are. This could even be argued to be a subconscious process, something picked up while learning the ritual in the first place. It could also explain how some circles are of limited use or appear to fail altogether; they were cast too short to be of much use. Potentially some interesting scenes there.

The logical extension of this is if they can be empowered during their configuration or creation, is there a way this could be used to greater impact? We’ve seen circles on the ceiling used to trap things below. What about a vertically mounted circle being used to close a passage or to create a battering ram that only hits supernatural creatures?

How about a dodecahedron, with circles carved into its faces, with a power source inside, allowing the circles to all expand at once? Certainly be a creative weapon in the right circumstances.