Hohenbaden Castle, built in 1102, and known locally as the Altes Schloss (Old Castle) was home from the 11th-15th century to the margraves of Baden. During its heyday in the 15th century, the castle had 100 rooms. In 1479 margraves moved their seat to Baden and the old castle started to decay. In 1599 it was destroyed by fire.
From the tower you have a good view over Baden-Baden and distant view of the Rhine valley. The courtyard of the ruins is also worth of seeing.
References:The stone church of Gamla Uppsala, built over the pagan temple, dates from the early 12th century. Due to fire and renovations, the present church is only a remnant of the original cathedral.
Before the arrival of Christianity in Sweden, Gamla Uppsala was the seat of Swedish kings and a ceremonial site known all over northern Europe. The settlement was home to royal palaces, a royal burial ground, and a great pagan temple. The Uppsala temple, which was described in detail by Adam of Bremen in the 1070s, housed wooden statues of the Norse gods Odin, Thor and Freyr. A golden chain hung across its gables and the inside was richly decorated with gold. The temple had priests, who sacrificed to the gods according to the needs of the people.
The first Christian cathedral was probably built in the 11th century, but finished in the 12th century. The stone building may have been preceded by a wooden church and probably by the large pagan temple.