As written, the Magic Circle ritual could make a very effective prison for your PCs -- but it would be a tricky thing to trap them in -- getting them to stand still while the circle is inscribed (one minute per square) would be a real trick. Not to mention the likelihood that the party includes some types that aren't included in the circle's magic (any of the PC's Fey instead of Natural?)
Once they get caught, though, their goose may well be cooked. Only someone unaffected can mar the circle and break it.
So, assuming the PCs are sluggish enough to wind up caught in the circle, just based on the rules they could try a couple of different things:
1. Teleportation or Planar travel out of the circle. This is not prohibited -- after all, one of the standard uses for a magic circle is to summon a demon into it. The circle does not interdict that sort of interplanar travel. Depending upon the party's resources, that may not be available for everyone, but it's a possibility.
2. Create a magic circle within the magic circle. Basically line the circle with another one that protects against the enemy. So, if a part of natural PCs is trapped in a magic circle by an undead enemy, they could protect themselves from the undead with a magic circle of their own.
Dispel Magic (power) only works on Conjurations and Zones, so in the RAW that won't help against the magic circle.
As DM, you have all kinds of other options. You might, for example, decide that the PCs could try to complete a skill challenge to unravel the enchantment of the circle, but you're right, if the PCs can do it, so can the bad guys.
Another thought: The ritual description reads: "An affected creature whose level is lower than your Arcana check minus 10 cannot pass through the circle, affect creatures through the circle's boundary, or affect the boundary in any way." That seems to indicated that you could affect objects outside the circle. The value of that would depend upon the area and what opportunities present themselves, but that might allow for indirect attacks -- bring the ceiling down on your enemies, etc. Might kill yourself in the process, but it's a contact sport, right?
-rg
