The decoration was probably cast using the new lost-wax technique.  First, a model of the object is made in wax.  Clay is then molded around it and the whole thing heated.  The melted wax then ran out, leaving the clay mold ready for the liquid bronze to be poured in.  The advantage of this method over the piece-mold technique is that small objects of irregular shape can be made since wax is relatively easy to shape.