Runkelstein Castle is a frescoed medieval castle that adds useful depth to a visit to Bolzano. It is a good stop when you want the local story told through objects, buildings, art, machinery, or archaeology rather than another broad city overview.
Allow enough time to read the displays instead of treating it as a rain-plan filler. The strongest reason to come is context: after visiting, the surrounding streets and monuments in Bolzano make more sense, and smaller details are easier to notice.