We give numerical evidence that in a two-dimensional lattice-dynamical XY model with quartic anisotropy a conventional continuous phase transition occurs. The formation of clusters in the angular displacement field separated by 90°walls is also demonstrated. Moreover, we give evidence that the critical dynamics of the order-parameter fluctuations is dominated by a central peak which splits at finite wave vectors. This phenomenon is traced back to damped traveling 90°walls. © 1976 The American Physical Society.