Isenburg is a ruined castle located high above the valley of the Zwickauer Mulde. Archaeological finds indicate that this site already existed in the 12th century. Little is known about the history of spur castle and its violent destruction. There are no verified, documented references. Oral traditions - the first account dates to 1738 - called the Isenburg a 'robber castle,' the 'Old Castle' and the 'Iron Castle', from which its present name is derived. The castle had been destroyed by the 14th century and was never rebuilt. Its ruins were probably used from the 15th to 17th centuries as shelters for the local population during times of conflict. Around 1750, the remains of the castle were demolished to build the stone church in Wildbach.
The recommended way to visit the castle ruins is by foot from Hartenstein railway station which takes about 30 minutes. Only about 200 metres from the station is Stein Castle. At this spot the route crosses the Zwickauer Mulde river to a point immediately below Schloss Wolfsbrunn. Here the route turns left and winds through the Poppenwald upstream and parallel to the Mulde. After a short, but steep, climb known as 'Gentle Henry' (Sanfter Heinrich), the path forks. Keeping right, the route passes the Wildbach Church on an easy forest path. Here there are signs to the ruins which are another 700 metres further on.
The left-hand fork leads via a wild and romantic narrow path on top of the river bank of the Mulde to the same destination. This path is called the 'Raubrittersteig' ('Robber Baron Climb') and is one of the most attractive walks in the Mulde Valley.
References: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.