In the Church of Saints Peter and Paul the relics of St. John of Polivotsky were transferred to the church by Tsar Kaloyan in the capital city of Turnovo in 1204. Subsequently, a monastery complex was formed around the church. It was built according to the will and priesthood of Anna-Maria (1221–1237), wife of Tsar Ivan Assen II. During the Ottoman rule, hundreds of manuscripts from the royal library were placed there. Many of them were destroyed in 1842 by the Greek ruler.
The church is dedicated to the Christian Apostles Saint Peter and Saint Paul. It follows the cross-domed design and has a single apse. The cella is divided into three naves by two rows of columns. The columns' capitals are decorated with plastic carving and tracery. The church has a high, massive iconostasis. According to the 14th-century account of Patriarch Evtimiy, the church and the surrounding monastery were built on the order of Tsar Ivan Asen II's (ruled 1218–1241) wife Anna.
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.