Altenberg abbey was founded in 1133 as a daughter house of Morimond Abbey and settled initially in the old castle of the Counts of Berg, Burg Berge, which the counts had left for Schloss Burg, but moved to the new purpose-built monastery in the valley of the Dhünn in 1153. It flourished sufficiently to undertake the settlement of a number of daughter houses of its own from 1143 to 1443.
In 1803 it was dissolved during the secularisation of Germany and fell into ruin. Starting in 1847 under King Frederick William IV of Prussia, a thorough restoration was carried out, and the restored church, known as the Altenberger Dom ('Altenberg Cathedral', although Altenberg was never the seat of a bishopric so this is not technically accurate), is now an interdenominational church used by both Roman Catholics and Protestants.
References:Ehrenbreitstein Fortress was built as the backbone of the regional fortification system, Festung Koblenz, by Prussia between 1817 and 1832 and guarded the middle Rhine region, an area that had been invaded by French troops repeatedly before. The fortress was never attacked.
Early fortifications at the site can be dated back to about 1000 BC. At about AD 1000 Ehrenbert erected a castle. The Archbishops of Trier expanded it with a supporting castle Burg Helferstein and guarded the Holy Tunic in it from 1657 to 1794. Successive Archbishops used the castle's strategic importance to barter between contending powers; thus in 1672 at the outset of war between France and Germany the Archbishop refused requests both from the envoys of Louis XIV and from Brandenburg's Ambassador, Christoph Caspar von Blumenthal, to permit the passage of troops across the Rhine.