My reading of the description of the magic circle spell is that normally an extra-planar being of one of the specified types couldn't walk into the cylinder:
The creature can't willingly enter the cylinder by nonmagical means. If the creature tries to use teleportation or interplanar travel to do so, it must first succeed on a Charisma saving throw.
…but it could presumably walk out of the cylinder if it was already standing inside at the time of casting. Because the inverted circle operates "in the reverse direction," an extra-planar being could walk into the cylinder if they didn't realize that it was there and then get stuck:
When you cast this spell, you can elect to cause its magic to operate in the reverse direction, preventing a creature of the specified type from leaving the cylinder and protecting targets outside it.
I don't mean summoning something into a magic circle, but more like putting one down as a sort of inter-dimensional flypaper or roach motel. Fiends check in, but they don't check out.
So what do you think, does my interpretation make sense?
Best Answer
Yes, it does.
When reversed, nothing states that the extra-planar creature can't enter, but it explicitly says it can't leave, the very definition of a trap for pests.
The main purpose of the reverse circle is to contain creatures you're summoning so it needs to let them in. Thanks to Mark Wells for the heads up.
Your extra-planar flypaper, besides the 1 hour short duration and the fact that there are easy to see glowing runes on the floor, is functional. But looking at these points make it not the best choice for a trap.