At the place where the Kamenický stream meets the Sázava river, you can visit the mysterious ruins of the royal castle demolished Kostelec. The ruins are part of a nature reserve and are freely accessible.
The castle was probably founded in the last third of the 13th century by King Wenceslas II, but the first written mention dates back to 1342, when it was held in pawn by Oldřich Medek of Valdek. Although Charles IV bought back the castle in 1356, he soon pawned it again and it had several owners over time. The last of them was Kuneš Rozkoš of Dubá from 1443, who looted in the surrounding area, and therefore the castle was besieged in 1449. It withstood this siege, but a year later, on May 18, 1450, it was besieged again and five days later it was conquered by the army under the command of Zdeněk Konopišťský of Šternberk, who became its new owner. In 1456, serfdom was abolished and he fortified it as one of his military strongholds. Later, he became a leading figure of the anti-Poděbrady opposition, and therefore the royal army captured and destroyed the castle in 1467.
The Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.