  Spiral waves in two-dimensional excitable media have been observed
experimentally and studied extensively. It is now well-known that the symmetry
properties of the medium of propagation drives many of the dynamics and
bifurcations which are experimentally observed for these waves. Also,
symmetry-breaking induced by boundaries, inhomogeneities and anisotropy have
all been shown to lead to different dynamical regimes as to that which is
predicted for mathematical models which assume infinite homogeneous and
isotropic planar geometry. Recent mathematical analyses incorporating the
concept of forced symmetry-breaking from the Euclidean group of all planar
translations and rotations have given model-independent descriptions of the
effects of media imperfections on spiral wave dynamics. In this paper, we
continue this program by considering rotating waves in dynamical systems which
are small perturbations of a Euclidean-equivariant dynamical system, but for
which the perturbation preserves only the symmetry of a regular square lattice.
