Cloister is said to block "transportation magic" and "plane shifting". What it apparently doesn't block are summoning and calling spells. I'm of the opininon that the spell that brought Celia to her broken talisman was a calling spell, since she stayed on the plane and since she felt she could be killed on the mortal plane (instead of reappearing on the Plane of Air, as would be the case if she was killed while the subject of a summoning spell). Since Gate is a calling spell, if I'm right about Celia's talisman, it should work to call creatures into the Cloister from outside.