A modular, prefabricated stair tower comprising a plurality of stairs, landings, handrails and hangers pre-assembled into a module configured such that it might be shipped and installed into a wind power tower tube section, with installation preferably occurring prior to shipment to the wind farm jobsite so that, when the tube section is erected vertically, authorized personnel gain immediate, efficient and economical access for ascending and descending large land based and offshore wind power turbine towers.