  The crystalline ground state of macroions confined between two neutral
parallel plates in the presence of their homogeneously spread counterions is
calculated by lattice-sum minimization of candidate phases involving up to six
layers. For increasing macroion density, a cascade of solid-solid transitions
is found involving various multilayered crystals. The cascade includes
triangular monolayer and buckled bilayer as well as rhombic, squared and
triangular phase structures.
