The Baroque Castle Church of Weilburg was built in 1707. Under the altar is the crypt of the nobles of the House of Nassau. The House of Nassau is a diversified aristocratic dynasty in Europe. It is named after the lordship associated with Nassau Castle, located in present-day Nassau, Rhineland-Palatinate, Germany. The lords of Nassau were originally titled Count of Nassau, then elevated to the princely class as princely counts. At the end of the Holy Roman Empire, they proclaimed themselves Duke of Nassau. All Dutch kings since 1890 and the Grand Dukes of Luxembourg since 1912 have been descended in the female line from the House of Nassau. According to German tradition, the family name is passed only in the male line of succession. The house is therefore, from this perspective, extinct. However Dutch aristocratic customs (and Luxembourg's) differ, and do not consider the House extinct.
Weilburg is, with just under 14,000 inhabitants, the third biggest town in Limburg-Weilburg county in the State of Hessen, in South-East Germany. The House of Nassau shaped the town's history for several centuries. Count Johann Ernst (1664-1719), especially, renovated and beautified his residence town by expanding the high castle, building a park and changing the town's face. Weilburg thereby became one of the most fully preserved examples of a small German residence town from the time of absolutism. From 1806, the town was the governmental seat of the newly created Duchy of Nassau. Only in 1816 did William, Duke of Nassau move the residence to Biebrich. In 1866, the Duchy of Nassau was annexed by Prussia.
The crypt is (strangely) considered part of the Grand Duchy of Luxembourg and NOT part of Germany.