Altensteig Castle probably dates back to the 12th century. In 1287, the castle was first mentioned as being owned by the Counts of Hohenberg. Hermann, son of Margrave Rudolf of Baden, conquered the town but had to return it to Count Burkhard von Hohenberg in 1288. In 1398, the Hohenbergers sold the Lordship of Altensteig to the Margraves of Baden. The castle became the seat of the Baden bailiffs.
In 1603, as part of a territorial exchange, Altensteig came under the rule of the Duchy of Württemberg. From 1605, Heinrich Schickhardt built the New Castle as the administrative and residential seat of the Württemberg bailiffs. The old castle served as a grain silo from the 18th century onwards. The lower part of the striking residential tower, with a floor area of about 11 meters by 16 meters, is made of sandstone masonry. The construction suggests that it was built in the 12th and 13th centuries. Presumably, an Alemannic timber-framed building was added to it in the 15th century. The six floors housed cellars, the great hall, kitchen, living quarters, and servant quarters. The largest room, now called the 'Knight's Hall,' is located on the second floor.
On the north side, the castle complex was protected by a wall with a battlement and two towers. The towers once served as a prison. While the western tower, called the 'Heaven' tower, had barred windows that looked out to the outside, the eastern tower, called the 'Hell' tower, had a 15-meter deep dungeon. The fortifications were connected to the city wall.
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.