  We study fourfold rotation invariant gapped topological systems with
time-reversal symmetry in two and three dimensions ($d=2,3$). We show that in
both cases nontrivial topology is manifested by the presence of the
$(d-2)$-dimensional edge states, existing at a point in 2D or along a line in
3D. For fermion systems without interaction, the bulk topological invariants
are given in terms of the Wannier centers of filled bands, and can be readily
calculated using a Fu-Kane-like formula when inversion symmetry is also
present. The theory is extended to strongly interacting systems through
explicit construction of microscopic models having robust $(d-2)$-dimensional
edge states.
