At the beginning of the 3rd century BC, the Celts invaded the Balkans, and a Celtic tribe called the Scordisci settled in the territory of modern Belgrade. Their fortified settlement was called Singidunum. According to one version, the name means "round city." According to another version, at the time of the Celts' arrival, this land was occupied by the Thracian tribe Sings, after whom the Celts named the new city. Besides the necropolises found in Karaburma and Rospi-Čuprija from that period, there is no other information about Singidunum under the Celts. According to archaeological excavations, the Celtic fortification was located in the area of modern Karaburma.

At the base of the walls of the Belgrade Fortress, remains of Roman castrum fortifications were discovered. In the 1st century BC, the Romans began their advance against the Illyrian tribes and the Scordisci from the territory of Macedonia. Gaius Scribonius Curio, proconsul of Macedonia, made a campaign deep into the peninsula, reaching the Danube. Little is known about the conquest of this part of the Balkans, but for several more years the local population tried to resist Rome. The proconsul of Macedonia, Marcus Licinius Crassus, suppressed the resistance of the Bastarnae here in 29 BC. It is not exactly known when these lands were transformed into the Roman province of Moesia. According to Dio Cassius, Moesia existed as a province already at the very beginning of the 1st century and was governed at that time by Cecina Severus.

At the beginning of Roman rule, the fortifications of Singidunum consisted of ramparts with palisades. Soon a castrum was built, measuring 560 meters in length and 350 meters in width. It was located on the site of the modern Upper Town and part of Kalemegdan up to Paris Street.

The importance of Singidunum as a military center of the region increased in the 3rd century when the Romans, due to invasions by the Goths and other barbarian tribes, abandoned Dacia and the border ran along the Danube. Singidunum thus again became a fortress on the Empire's border. The famous road Via Militaris also passed through the city, along which a series of fortifications were built. After the division of the Roman Empire in 395 AD, Singidunum became part of the Eastern Roman Empire.

The division of the empire occurred during the Great Migration Period. Being a border fortress, Singidunum became a target for many tribes invading the Eastern Roman Empire. In the first half of the 5th century, it repeatedly withstood sieges, but in 441 AD the Huns managed to storm it, after which the city was burned. In 454 AD, Byzantine (Eastern Roman) troops managed to retake it, but soon it was captured by the Sarmatians. In 470 AD, Singidunum was taken by the Ostrogoths. In 488 AD, it was seized by the Gepids, but in 504 AD the Ostrogoths recaptured the city. A few years later, according to a peace treaty, they returned it to Byzantium. In 512 AD, the emperor decided to settle the Heruls there to protect the empire's border from attacks by the Gepids from the other side of the Danube.

Under Justinian I, powerful stone fortifications were erected around Singidunum. In 584 AD, the city was captured and plundered by the Avars and Slavs, who, however, soon left it, and Singidunum was again occupied by a Byzantine garrison. At the turn of the 6th–7th centuries, the city was a stronghold of the Byzantines during Emperor Maurice's campaigns against the Slavs and Avars. In 602 AD, the Avars again stormed Singidunum and plundered it. Around 630 AD, Serbs settled in its vicinity. It should be noted that the repeatedly destroyed city had by then lost its military significance. After that, for more than two centuries, it is not mentioned in sources. Only in 878 AD, in a letter from the Pope to the Bulgarian prince Vladimir-Michael, is the city mentioned again, but already under its Slavic name Belgrade. At that time, it was part of the First Bulgarian Empire. Belgrade was taken from the Bulgarians by the Hungarians, but after some time Bulgaria managed to regain the city. In 1018 AD, the city became part of Byzantium and again played the role of an important border fortress of the empire.

In 1040 AD, an anti-Byzantine uprising broke out in the Pomoravlje region under the leadership of Peter Delyan. Among the cities captured by the rebels was Belgrade, where Delyan declared himself the grandson of the Bulgarian Tsar Samuel and was proclaimed Bulgarian tsar under the name Peter II. In 1041 AD, the uprising was suppressed by the Byzantines. Throughout the 11th–12th centuries, the city was a scene of fierce struggle between Byzantium and Hungary. Besides the armies of these states, it was several times ravaged by crusaders passing through the area on their way to the Holy Land.

In 1284, the King of Serbia, Dragutin, received the region of Mačva with Belgrade from his Hungarian king Ladislaus IV as a fief. He actively settled Serbs there, and the influence of the Serbian Orthodox Church grew in the city. New construction was actively carried out. In 1319, a few years after Dragutin's death, the Hungarian army captured and destroyed the city. Throughout the 14th century, it was a border outpost that Hungarian kings viewed as an obstacle preventing Serbia from expanding northward.

After the arrival of the Turks in the Balkans and the Battle of Kosovo Field, the Hungarians, seeking to defend the Danube with foreign hands, handed Belgrade over to the Serbian despot Stefan Lazarević. He rebuilt the city and erected powerful fortifications there. Belgrade was divided into two parts — the Upper Town and the Lower Town.

Stefan's heir Đurađ (George) Branković was forced to return the city to Hungary. Following the model of the Belgrade Fortress, he built fortifications in Smederevo. Meanwhile, Belgrade under Hungarian rule quickly lost its economic and cultural role. Moreover, Hungarian national policy had a negative impact — King Sigismund settled Hungarians in the city, and Serbs were forbidden to enter its central part.

For the Ottoman Empire, capturing Belgrade was an important task, as the fortress blocked the way to Hungary and prevented the Turks from conducting offensive campaigns in that direction. In 1440, the Turkish army of 100,000 men under Sultan Murad II besieged Belgrade. They failed to take the city, but on the top of Avala, on the site of the Serbian fortification Žrnov, the Turks built a fortress and stationed a large garrison there, which became a stronghold for subsequent attacks on Belgrade. In 1456, the Turks made another unsuccessful siege of the city. Fierce battles continued around it until the end of the century. In 1521, Sultan Suleiman captured Belgrade.

After the defeat at Vienna in 1683, the Turks began to lose their European possessions, and in 1688 the city was captured by the Austrians. They immediately began construction of a new modern fortress designed by András Kőnig. Two years later, the Turkish army recaptured Belgrade. During the siege, one of the towers in the Upper Town castle was damaged by a cannonball. A fire broke out in the fortress, causing the powder magazine to explode. More than a thousand people died, and the castle of Despot Stefan was destroyed. After capturing the fortress, the Turks continued construction of fortifications according to Kőnig's design, as he entered their service.

For several years, the city was a Turkish border fortress, but in 1717 it was again taken by the Austrian Empire's troops. The fortress was rebuilt according to a modern design, and the fortifications surrounded the entire city. Under Austrian rule, Belgrade experienced a brief revival. Many new buildings were constructed, and trade flourished. Among its inhabitants were many Hungarians, Germans, French, Czechs, and others. As a result of another war with Turkey, Austria was forced to cede Belgrade after the peace treaty signed there in 1739. The Turkish garrison demolished the outer fortifications, barracks, and other buildings constructed by the Austrians. Several Christian churches were converted into mosques.

During another war with the Ottoman Empire, the Austrians captured the city in 1789 but left it after signing the Treaty of Sistova. Meanwhile, Belgrade was closed to the Janissaries, who were forbidden to visit it. After the death of Mustafa Pasha in 1801, the Janissaries managed to seize power in the city and its surroundings. Their lawlessness and terror led to the First Serbian Uprising.

The uprising that began in 1804 had among its goals the liberation of Belgrade. In 1807, the Serbian army under the leadership of Karađorđe managed to capture the city. The insurgents found it in poor condition; the city was in decline, and many buildings were completely or partially destroyed. Karađorđe proclaimed Belgrade the capital of the country and began its restoration. The development of the capital was interrupted by the defeat of the uprising in 1813. The Turkish garrison that occupied Belgrade carried out repressions, which led to the Second Serbian Uprising, which began in 1815. Its leader Miloš Obrenović, with Russian support, managed to achieve autonomy for Serbia within the Ottoman Empire.

On April 19, 1867, after lengthy negotiations, the Turkish garrison left Belgrade, and the city once again became the capital of Serbia. From that moment, the fortress lost its military significance. During World War I, all buildings inside the fortifications were destroyed, and the walls and towers were seriously damaged. After World War II, the fortress was transformed into a tourist attraction and a recreational area for the residents of Belgrade. Currently, cultural and sports events are regularly held on its territory and in the adjacent Kalemegdan Park.

Sources:

https://ru.wikipedia.org/wiki/Калемегдан