Twenty-four members of the House of Kettler and six members of the Biron family are buried at this location. Initially, the remains of dukes and their family members were placed in a vault under the chapel, the sarcophagi were later moved to the basement rooms under the new Jelgava Palace.

The vault contains 21 metal sarcophagi and nine wooden coffins, all adorned and decorated with fine engravings and ornamental details. The oldest pewter sarcophagus belongs to Sigismund Albert, the son of Duke Gotthard. Sarcophagi from earlier periods (the end of the 16th-century and the beginning of the 17th-century) are simpler with only a few ornaments and coats of arms.

The most sumptuous pewter sarcophagi belongs to Duchess Elisabeth Magdalene, her husband Duke Friedrich, and Duke Wilhelm.

Another group of stylistically similar sarcophagi in Baroque style belong to Duke Jacob, his wife Louise Charlotte, and their son Karl Jacob. They are richly adorned with acanthus leaf motifs, reliefs, and coats of arms of Courland-Semigallia. The last member of the Dukes dynasty who was laid to rest here was Princess Charlotte Friederica.

The vault has sustained severe damages over the course of history and has been looted several times. The sarcophagi have been restored and are a permanent exhibition of the Palace Museum.