SRD:Repel Vermin

An invisible barrier holds back vermin. A vermin with Hit Dice of less than one-third your level cannot penetrate the barrier.

A vermin with Hit Dice of one-third your level or more can penetrate the barrier if it succeeds on a Will save. Even so, crossing the barrier deals the vermin 2d6 points of damage, and pressing against the barrier causes pain, which deters most vermin.