Kupinik or Water City is a medieval fortress in the village of Kupinovo in Srem, the capital of the last Serbian despots. The fortress was built on a small island in the Sava, on the edge of today’s special nature reserve Obedska bara. Although it is a cultural monument of great importance, the fortress has not been explored to this day. We know about it from historical records and two charters from the second half of the 14th century issued by King Žigmund of Luxembourg. The fortress was built as a military-border fortification of the Hungarian king, only to become the residence of Serbian despots in the 15th century. Despot Stefan Lazarević received the capital Belgrade and several other cities and estates from the Hungarian king, including Kupinik. After the death of despot Stefan, his successor Đurađ Branković returned all the estates to the Hungarian king, but he got some again, so Kupinik became an important residence of Serbian rulers and their heirs. After the death of despot Đurađ in 1456, his grandson Vuk Grgurević, better known as the Zmaj Ognjeni Vuk, passed into the service of the Hungarian king Matija Korvin and raised the Kupinik estate to the rank of capital. It is especially important to point out that the Hungarian king Žigmund was in friendly relations with the despot Stefan Lazarević who he even introduced to the famous organization of knights in Europe, the knightly order of the dragon.

Kupinik is one of the better preserved medieval fortifications in Vojvodina. The fortress has an irregular quadrangular base with protruding towers of various shapes reminiscent of the towers of the city of Smederevo and the Belgrade tower of Nebojsa. The fortress was entered through the fifth tower on the northeastern rampart, through the gate with a drawbridge. The towers were about 16m high connected by massive ramparts, and there was a moat around the ramparts. The fortress was built of crushed stone, lined with bricks on the outside. There were residential buildings inside the fortress, but there is no information about them, because no archeological research of the fortress has been done until today. When the fortress was taken over by Serbian despots at the beginning of the 15th century, it became an artillery fortress with cannon towers from a fortification for warfare with cold weapons.

Despot Đurađ built a church near the fortress, which according to many researchers is the oldest Serbian Orthodox church north of the Danube and Sava. The despot connected his court church dedicated to St. Luka, otherwise the protector of his capital Smederevo, with the Kupinik fortress by a bridge. 

In 1521, during the campaign of Sultan Sulejman on Belgrade, the Ottoman army built a bridge near Šabac and crossed to the Srem. The Turks then conquered many fortifications, including Kupinik. The last lords of the town of Kupinik were the widow of Jovan Branković, Jelena and her son from another marriage, Stefan Berislavić. Before the invasion of the Turks, despot Jelena withdrew deeper into Hungary with her underage son.

Kupinik, a fortress on the Sava, is waiting to be renovated and opened to visitors. Under the layers of weeds and grass, the centuries are silent and keep the memory of one tragic, but still glorious time of the last Branković.