A hollow module useful for retaining wall structure or the like defined by an upstanding surrounding wall formation presenting front, rear and opposed side faces having generally planar upper and lower surfaces, and a substantially constant wall thickness throughout the extent of any given face, the lower planar surface in the regions of the opposed side faces presenting at least a pair of opposed depending lug formations projecting therebelow of dimensions such that when measured in the front to rear direction the extent of same are always less than the front to rear extent of the opposed side faces so as to define with the lower planar surface recess formations for selective registration over the upper planar surfaces of other like or compatible modules whereby such modules are selectively interlockingly interengaged; and wherein suitable slabs are provided with depending lug formations projecting therebelow for disposition upon upper planar surfaces of such modules with the lug formations projecting therew
