Provided is a displaceable maintenance device, to a displaceable assembly device, to a method for servicing a fastening element, and to a method for servicing and/or assembling a fastening element of a flange of a wind power installation tower. Provided is a displaceable maintenance device for a wind power installation tower, comprising a mobile unit which is specified for moving the maintenance device along a flange of the wind power installation tower; a first maintenance unit having an impact element for impacting a fastening element, in particular a flange connection element, and a noise-receiver unit for detecting a noise generated by the impacting; a control unit for evaluating the noises detected by means of the noise-receiver unit, said control unit, based on the evaluation, being specified for assigning a maintenance status to a fastening element.