It is the best-preserved and largest ensemble of Baroque buildings in Croatia and consists of a Habsburg star fort built on the right bank of the River Drava.

While the fortifications have largely been removed, the fort's interior core remains intact and is now home to churches, museums, schools and other public buildings, as well as numerous bars and restaurants.

Tvrđa sustained significant damage during the Croatian War of Independence during the 1990s and was featured on the 1996 World Monuments Watch List of Most Endangered Sites.

Between 1526 and 1687 Osijek was ruled by the Ottomans, who did not change the layout of the settlement in any substantial way but introduced Islamic places of worship, giving the area an Oriental appearance.

[5] The bridge, which connected Osijek and Darda, took the form of a wooden road on piers and was approximately 7 kilometres (4.3 mi) long and 6 metres (20 ft) wide.

[2] Development of the military settlement at Tvrđa started in 1687 when the Habsburg armies drove the Ottomans out of the city during the Great Turkish War.

[7] The chief commander of the Imperial army, Louis William, Margrave of Baden-Baden, saw Osijek as a location of exceptional strategic importance in the war against the Ottomans.

[3] The original plan for Tvrđa was drafted because of the need to reinforce the town walls, but did not include provisions to redesign the interior and envisaged largely uncontrolled development.

[2] Starting in August 1712, Austrian engineers, supervised by the fort's commander, General Johann Stephan von Beckers,[10] built barracks, staff headquarters, churches and monasteries, surrounded by system of moats, bastions and gun positions, respecting Gosseau's design.

The construction of the outwork on the opposite bank of the River Drava, designed to offer protection from the north and to serve as a bridgehead, was completed by 1721.

[2] Based on the 'ring model', the fortifications took up an area of 80 hectares (200 acres),[3] making Tvrđa the largest fortress on the border with the Ottoman Empire.

[7] The Yugoslav People's Army maintained a garrison and a military hospital in Tvrđa, but in the 1980s these buildings were gradually being abandoned, and adapted into ateliers for local painters and sculptors.

It played the role of the almost identical town of Theresienstadt in Czechoslovakia, which had been converted to a concentration camp known as the "Paradise Ghetto", to which prominent Jews were sent by the Nazis.

[1] After the fortress's military importance decreased at the end of the 19th century, Tvrđa became a center of administrative, educational, cultural, and scholarly life in Osijek and the entire region.

[21] The first school in Osijek was organized at Tvrđa; the first scholarly curriculum was introduced in 1707, to be later expanded and renewed, and the first printing press started working in 1735.

[21] The Faculty of Agriculture of the Josip Juraj Strossmayer University of Osijek has occupied the former general headquarters since 1995 after its previous site was destroyed in the Croatian War of Independence.

[14][27] The former town museum and archives building today houses the State Directorate for Monument Protection, a department of the Croatian Ministry of Culture.

[24] "I have seen many European towns, but have never found an identical development whereby an existing urban nucleus was turned into a fortification, or a similar town-planning solution".

[28] During the 1991–95 conflict in Croatia, 90 per cent of the buildings in Tvrđa were damaged to some extent and the fort was featured on the 1996 World Monuments Watch List of Most Endangered Sites.

[29] The building of the general headquarters, dating from 1726, and the ground plan of the fortress were depicted on the reverse of the Croatian 200 kuna banknote, issued in 1993 and 2002.