Those inside the circle are protected from illusions. Illusions outside the circle are unaffected but any that broach the circle are revealed.
For example, the group sees that the tunnel ahead of them has completely collapsed. The mage puts up a Circle of Protection vs. Illusions and is of high enough rank to affect everyone in the group. He then moves forward and sees that there is really no collapsed tunnel. Beyond the circle of protection, the tunnel still appears to be collapsed but, as the mage (and thus, the circle) move forward, an open passage is revealed. |