Here we demonstrate theoretically and experimentally that a two-dimensional time-reversal invariant photonic topological insulator supports zero-dimensional defect modes that are protected to lie at the center of the band gap. The fact that they lie at mid-gap implies that they are as spatially confined as possible for a given amount of disorder. Since defect modes in photonic crystal slabs and fibers require maximal confinement and are highly sensitive to disorder, topological protection may provide a route to engineering robust and efficient photonic cavities.