The Monastery of the Cross was built in the eleventh century, during the reign of King Bagrat IV by the Georgian Giorgi-Prokhore of Shavsheti. It is believed that the site was originally consecrated in the fourth century under the instruction of the Roman emperor Constantine the Great, who later gave the site to king Mirian III of Kartli after the conversion of his kingdom to Christianity in 327 AD.
Legend has it that the monastery was erected on the burial spot of Adam's head—though two other locations in Jerusalem also claim this honor—from which grew the tree that gave its wood to the cross on which Christ was crucified.
Due to heavy debt the monastery was sold by the Georgians to the Greeks in 1685. It is currently occupied by monks of the Greek Orthodox Patriarchate of Jerusalem.
The remains of the crusader-period monastery forms a small part of the current complex, most of which has undergone restoration and rebuilding. The crusader section houses a church, including a grotto where a window into the ground below allows viewing of the spot where the tree from which the cross was (reputedly) fashioned grew. Remains from the 4th century are sparse, the most important of which is a fragment of a mosaic. The main complex houses living quarters as well as a museum and gift shop. The monastery library houses many Georgian manuscripts.
References:Sigmaringen Castle was first mentioned in the year 1077 in the chronicles of Petershausen monastery. The oldest parts of the castle are concealed beneath the alterations made during the 17th and the 19th centuries. The secret of the earliest settlement built on this defendable rock will never be fully revealed: large-scale excavation work would be necessary, which the extensive land development renders impossible. Judging from the many Roman remains unearthed in the area around Sigmaringen, the 12th century keep known as the 'Roman Tower' could be traced back to a Roman predecessor.
The castle remains that have been preserved (gate, great hall and keep) date back to the Staufer period around 1200. The castle remains were integrated into subsequent buildings. The foundations of the castle buildings are to a large extent identical to the surrounding castle wall.
These remains give us a good idea of how the castle might have looked during the 12th century.