Method and apparatus for forming cast members having curved or flat walls. Steps are provided in the cast member by placing hollow plastic step inserts into holders provided along the inner core of a mold assembly or mounting inserts on reciprocating pins or mounting inserts to a marginal portion of the inner core. The cast material is then poured into the mold. The cast member, when set, is withdrawn from the mold assembly. In one embodiment, a holder portion of the step insert is broken away, leaving the step inserts embedded in the cast member to receive legs of manhole steps. The inserts are provided with flanges, preventing leakage of the cast material into the mold assembly. Similar seals are provided by other types of cylindrical-shaped inserts for use in wetcast and drycast fabrication methods. Mounting assemblies are also provided for holding inserts for embedment in planar walls.