The old Old Market of the city, today is the architectural monument of Calvin Square. Here stands the only remaining medieval monument of the city, the Reformed castle church, which dates from the 15th century. At the end of the 20th century and in the 16th century. It was built in the early 20th century in Gothic style. It gained its present form after its transformation in 1547. After the great earthquake of 1802, in 1829 a new baroque bell tower was built beside it.
The church is surrounded by an oval castle wall with gaps, which have been restored. It was surrounded by a second fortress, which was probably partially damaged during the Turkish-Tartar riots of 1558-1661. The so-called German school, the modern cantor and bell tower, was built on a section of it. The arched gate of the building may have been the former gate of the castle. According to oral tradition, the depression of what is now Castle Street, the so-called Castle Gate, was a fortification around the fortress.