Schloss SchallaburgSchloss means "castle" in German. Schloss Schallaburg is located between and to the south of Loosdorf and Melk. While it's not on the Danube River, it's close enough to deserve mention. Schallaburg Renaissance Castle dates back to the 11th century. Originally a defensive stronghold, it was later redesigned as an artistic seat of power.
Melk AbbeyDominating the skyline of Melk, the impressive Stift Melk (or Melk Abbey) sits atop a hill overlooking the Danube River. It has been run uninterrupted by Benedictine monks since 1089. The Baroque building you see today was constructed between 1702 and 1736. With 500 rooms and 1365 windows, it is impressive to look at. While much of the Abbey is private - housing about 20 monks and a school of over 500 students - you can still walk through the museum, marble hall, library, and church for a fee of 9,50 Euro, (11,50 Euro with guided tour).
Burgruine AggsteinThe ruins of Castle Aggstein are located on the top of a steep hill on the right bank of the Danube. Originally built in the 12th century, the castle fell into ruins over the years. Today, the ruins can be rented for weddings and medieval banquets. Entrance fee: 6,50 Euro.