The Cathedral of Graz (Grazer Dom) was built in the 1460s under Emperor Friedrich III, who used it as his court church. The Gothic exterior is relatively plain; the interior is the reason to go in. The choir stalls are 15th-century Gothic, the reliquary cabinet in the sacristy still holds the personal relics Friedrich kept nearby, and the overall atmosphere is one of authentic medieval piety rather than tourist-polished grandeur.
Next to the cathedral stands the Mausoleum of Ferdinand II, and together they form a dense cluster of Habsburg religious architecture in the narrow streets east of the Hauptplatz. Worth 30 minutes of your time. Free entry.
Burgring/Kaiserstrasse area, directly south of the old town centre.