Tvrđa (Citadel) is the old town of the city of Osijek in Croatia. 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. Tvrđa has been described by the World Monuments Fund as "a unique example of an eighteenth-century baroque military, administrative, and commercial urban center". The center of medieval Osijek was on the banks of the River Drava where Tvrđa now stands. The site was home to the Romanesque church of the Holy Trinity. 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.
During the Ottoman period, Osijek was internationally known because of the Suleiman Bridge. The bridge, which connected Osijek and Darda, took the form of a wooden road on piers and was approximately long and wide. The bridge was rebuilt during the rule of Suleiman II. 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. He urged the repair of the city walls, and proposed construction of a new fort according to Vauban's principles of military engineering.]]
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. New plans for a fort on the right bank of the River Drava were drawn up by Maximilian Gosseau de Henef. Gosseau took over planning of the fort when construction was already under way. built barracks, staff headquarters, churches and monasteries, surrounded by system of moats, bastions and gun positions, respecting Gosseau's design. The design followed the model of lowland Dutch military fortifications of the period. Construction of the inner town was completed by 1733, and in 1735 three additional northern bastions were completed, along with a post office, the fort's construction office and a hospital.]]
Gosseau's plan left space for churches to be built where mosques had once stood. Initially, converted mosques were used as churches, but Franciscans started to build a Baroque church in 1709 and it was consecrated in 1732. In 1725, the Jesuits commenced construction of the parish church of St. Michael (), following the construction of their own monastery. This church was in use after 1734, despite being incomplete.
In the mid-18th century there were reportedly more than 35 inns in Tvrđa, estimated to an account for one in three of the fort's buildings.
19th and 20th century
[[File:Vodena vrata Tvrdja.jpg|thumb|right|The 'water gate' () is the only surviving gate out of four that were originally built.
From February to June 1986, the fortress town was used as a filming location for the epic American miniseries War and Remembrance. 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. Stars Jane Seymour, John Gielgud and Robert Stephens filmed throughout the town.
The fort sustained considerable damage during the Croatian War of Independence, which lasted from 1991 until 1995. The war brought structural damage from collapsing roofs, walls and floors. 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. The Faculty of Food Technology has been relocated to a building that served as the first military hospital in Osijek, from the mid-17th century until the beginning of the 1990s. Other present-day educational institutions in Tvrđa include the II and the III Gymnasium, Franjo Kuhač Music School (former Roman Catholic Seminary), Jesuit Classical Gymnasium (former logistics barracks built in the mid-18th century), and the Secondary School of Economics (former grammar school for girls). The fort interior is now a centre of Osijek's nightlife. There are numerous bars and restaurants in Tvrđa. The fort hosts the Museum of Slavonia, the largest general-type museum in Croatia, located in Tvrđa since 1946. The former town museum and archives building today houses the State Directorate for Monument Protection, a department of the Croatian Ministry of Culture. 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.
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.
The Agency for Restoration of Osijek Tvrđa () was established in 1999. Its stated goals are protection, restoration and revitalization of Tvrđa. The restoration process aims to preserve architectural, historical and aesthetic qualities of Tvrđa in full accordance with the restoration principles set by the International Council on Monuments and Sites, while maintaining its multifunctional character. International cooperation is also envisioned, in particular with the Council of Europe. The Agency is jointly funded by the Government of Croatia, Osijek-Baranja County and the City of Osijek.