The kinetics of cation ordering in complex crystal structures, such as minerals and ceramics, is discussed with regard to the applicability of time-dependent Ginzburg - Landau theory (TDGL). Deviations from the predictions made by TDGL may arise due to partial conservation of continuous and non-uniform order parameters at low temperatures. Under these conditions the movement of areas with large order parameter gradients (`walls') provides a local alternative to the continuous ordering kinetics described by TDGL. A simple phenomenological description of order parameter kinetics taking these constraints into consideration can be obtained. The underlying assumption is that the geometry of spatial order parameter variation close to equilibrium resembles that of a phase-separated system. The resulting kinetics differs substantially from the predicted TDGL behaviour.