The Southeast tower of the parish church built on to the side chapel is consecrated to the Holy Cross and is named after its founder Franz Josef Keuffel of Ullberg.
The Keuffel family came from Swabia and followed the Fürstenberg family on their way to Weitra: Two brothers worked here, Anton Keuffel (died 1748) and Franz Josef Keuffel (died 1765). They were ennobled and were allowed to add to their family name the title of nobility "knights of Ullberg". Franz Josef held the important office of a bailiff and seems to have been rather wealthy. He had, inter alia, an extensive private library, which he bequeathed to the Fürstenbergs, and thus after Keuffel's death the library was added to the extensive manorial book collections. Keuffel's books are still preserved in the castle library and haver been identified. Franz Josef Keuffel of Ullberg made rich endowments for the poor as well as for church buildings and institutions in the immediate and wider surroundings of Weitra. He had the beautiful chapel of Schagges and a large shrine at Brühl built. But his most important legacy was "Keuffel's benefice" to the parish church of Weitra: This capital yielded a 5%-interest in the amount of 300 guilders per year, which were determined for a resident priest, who could make his living by this wage. This priest had this to fulfill certain obligations; so each week he had to read five masses for the founder and his "friendship". It was for this purpose that Keuffel's chapel was built on the site of the former sacristy. Already in 1754 the vestry was moved behind the high altar, and this is how at the southside of the tower the side chapel to the Holy Cross was built. It was finally consecrated in 1761. A fancy grating decorated by the coat of arms of the founder separates the chapel from the church. Its walls are lined with delicate stucco work. Besides of ornaments it also contains the letters FJKVU, the initials of the founder's name Franz Josef Keuffel von Ullberg. In some ways a reminiscence to the Pope's altar in St. Peter's Basilica in Rome, the big canopy altar is carried by four gnarled oak logs. It can be assumed that the artistic decoration of the chapel was designed by Balthasar Moll and has been manufactured in his workshop. A bell which belongs to Keuffel's benefice was cast in 1762 by Ferdinand Vötterlechner. It was bought to ring in the masses donated by Keuffel, and hangs now as an item of loan in the tower of the Holy Spirit Church. In the course of the ecclesiastical reform carried out in our country under Emperor Joseph II (1780-90) Keuffel's benefice was transferred to the castle and used for financing of the Court chaplain. In the wake of the great inflation after the First World War all monetary assets of the foundation lost their value, and thus Keuffel's foundation disappeared. What remains are solely the buildings, in particular the chapel.