The Šabac Fortress (originally Zaslon or Sava, Ottoman name Bogur Delen (Böğur Delen), meaning "flank breaker") is a fortification in present-day Šabac on the bank of the Sava River. Today, only the southern half of the fortress remains, with two round towers and a rampart, as the northern half was washed away by the Sava.

The name Šabac was first mentioned by Antonio Bonfini, a Hungarian court historian, who believed it was a Turkish word meaning something amazingly beautiful. There are no preserved records about what the first fortress on the bank of the Sava looked like, around which today's Šabac was founded. In ancient times, when Sirmium, now Sremska Mitrovica, was the capital of the province of Pannonia, the settlement on the Sava near Šabac could have been a river port and castle for communication with Sirmium. Whether the fortress was built on the foundation of another building has not yet been proven. In the 19th century, the writer and traveler Felix Kanitz was among those who believed that the Šabac fortress was built on the foundation of a Roman castle. About the place on the riverbank and the fortress he personally saw, he wrote: "The village of Zaslon, inhabited only by Christians, and the last Šabac, were created on terrain about 85 meters high, separated from the fortified Muslim 'Sebaz' by a vast and marshy 'Šabac field.'"

The fortress was built in 1471 by Isa-beg Isaković, one of the first Serbian statesmen to convert to Islam. Construction was initiated by order of Sultan Mehmed, and it was named Bogur Delen (Böğur Delen), meaning "flank breaker," clearly indicating its purpose to attack the flank in case of an invasion towards Belgrade. The fortress itself, located in the center of the enclosed area, preserved not only the foundation of the Roman castle but also a significant part of the roofing material, apparently of Roman origin. It was a quadrilateral deviating from geometric regularity on the northeast side, with protruding round towers at the corners and a rampart. "The Šabac fortress closely resembles Fetislam and Zemun, Turkish fortifications on the Danube, with a quadrilateral base and round towers. The fortress has a base in the form of a regular quadrilateral with one round tower (external diameter about 10 m) at each of the quadrilateral's vertices."

Numerous drawings, woodcuts, copper engravings, lithographs, and old maps tell about the appearance of the Šabac fortress. On a 1562 map, the fortress is mentioned for the last time under the name Zaslon. Six years later, on the first topographic map of Hungary, compiled by Lazar, secretary to the archbishop of Ostrigon Toma, published in Ingolstadt in 1528, Šabac was drawn on the Sava under its current name.

The oldest known drawing of the Šabac fortress is a woodcut image featured in Hartmann Schedel's World Chronicle of 1493. Knowledge about this is provided by Karl Schuchardt's book, in which he writes: "How far the method of fortifying a royal castle spread in the Middle Ages is shown by the Šabac fortress on the Sava, depicted in Hartmann Schedel's World Chronicle of 1493. It gives the impression of a reconstruction of the Dorestad plan: inside, an elongated rectangle divided into a larger and smaller courtyard. Outside along the city wall is empty, and everything is made of wood. Instead of a wall, a fence woven from vines is everywhere. Only numerous round towers at the corners, which are medieval additions, seem to be made of stone."

Like many other fortresses, the Šabac fortress was primarily built as a defensive fortification in case of enemy attack. Five years after the Turks built it, Hungarian King Matthias Corvinus conquered the Šabac fortress and turned it into an important defensive point of Hungary against the Ottomans. However, battles around Šabac, Belgrade, and Avala continued until 1521, when the Ottoman army first captured Belgrade and then the Šabac fortress, killing the entire garrison. On this occasion, Sultan Suleiman II said: "This is one of the cities I have conquered. It should be rebuilt." The Sultan's order was carried out, and he personally directed and supervised the works. An inner fortification and a pontoon bridge over the Sava were built. The appearance of Šabac fortified in this way is depicted in a copper engraving by Jovan Landrart from the second half of the 17th century.

The name of the Romanian duke Vlad Țepeș, better known in history as Count Dracula, is connected with the Šabac fortress in the struggle for liberation from the Turks. Although Duke Țepeș was known as a great hero, he ended up imprisoned by King Matthias Corvinus because of his cruelty and great hatred of the Turks. In Corvinus's army, besides Vlad Țepeș, there was also Vuk Grgurović, better known as the Fire Dragon Wolf. People still tell stories and legends that the Romanian duke Țepeș impaled a thousand Turks in the Šabac fortress and sent them down the Sava on rafts. When he returned to Romania, he was killed the same year. His beheaded body was buried in a monastery on Snagov Island, and his head was sent to Sultan Mehmed II. According to legend, the restless spirit of Vlad Țepeș still roams the battlefield and fortress on the Sava.

Before the First Serbian Uprising, the Šabac fortress alternated between Austrian and Turkish control. At that time, the city had two parts: a stone city and an outer earthen city, fortified with trenches and palisades. The rebels led by Karađorđe captured it in 1807, but six years later the Turkish garrison returned to the fortress and remained there for 54 years.

At the beginning of the 20th century, the Šabac fortress was destroyed due to a gunpowder explosion. Šabac is a cultural monument of great importance. Restored and renovated, it is the face of Šabac. Within its centuries-old walls, on the bank of the Sava, a wide variety of cultural events, festivals, and manifestations are held to popularize Šabac, its traditions, and culture.

Sources:

https://sr.wikipedia.org/wiki/Šabačka_tvrđava

https://tvrdjavesrbije.rs/tvrdjava-stari-grad-sabac/