The royal crypt is under the choir. A small bronze plaque in the floor of the choir marks the burial place. The crypt itself can be accessed from the south on the outside and is open on 1st of September every year, the day of Princess Eugenia’s death. In the front section of the crypt are the metal coffins of the last princely couple, Prince Frederick William Constantine and Princess Eugenia.
The heart of Princess Eugenia, who by request of her mother was first kept in the private chapel of Palais Leuchtenberg in Munich in an urn, has been stored in the right prayer chapel in the lower storey of the tower since 1952. There are also glass windows there on which the praying princess and her role model, St. Elizabeth of Hungary, are depicted.
In the left prayer chapel is the only known depiction showing the princely couple together, surrounded by the motto "Everything with God, nothing without Him". Another relic of the princess can be found in a radiating monstrance (not on display) made of gold-plated silver: a golden earring with real pearls and a gemstone, which belonged to Eugenia.