The Smederevo Fortress in the eponymous city is located at the confluence of the Jezava and Danube rivers and was built in the second quarter of the 15th century. The fortress is a classic water fortress (it is surrounded by the Danube and Jezava rivers, and to the south by an artificial rampart connecting the two rivers), which makes it unique in Serbian medieval architecture. Its design was modeled after Constantinople and its walls. The fortress covers an area of 10 hectares without the outer walls and towers, and 14.5 hectares with them, making it one of the largest fortresses in Europe. The preparation for construction was taken very seriously, with stones, lime, and wood brought from various regions. Everything was used: stones from the ruins of destroyed fortresses Magrum, Viminacium, and Areus, even tombstones. The construction of this monumental structure took two years.

Here, besides the towers, there was the prince's court, barracks, a church, and other buildings of various purposes. The fortress proved its strength during the Ottoman sieges:

In 1439, 130,000 soldiers of the Turkish army, led by Sultan Murad II, began the siege of the fortress. It withstood cannon fire and attacks, but hunger, which followed after three long months of constant siege, forced the fortress to surrender.

In 1453, the attack by Mehmed II (1451-1482), who captured Constantinople the same year, was repelled.

In 1456, another Turkish siege was repelled.

Ultimately, the city was surrendered without a fight in 1459, marking the formal end of independent Serbia in the fifteenth century.

The high quality of the fortress's design is evidenced by the fact that from the completion of construction in the mid-15th century and the addition of four artillery towers by the Ottomans in 1480, until the end of their use in the second half of the 19th century—after it was handed over to Prince Mihailo Obrenović of Serbia in 1867—the fortress was operated without reconstruction.

In the twentieth century, the fortress was partially destroyed during the two world wars due to bombings and shelling, but nevertheless retained its monumentality. Today, the large fortress city is mostly in ruins and continues to deteriorate, while restoration work has been carried out in the small town area, partially restoring its former appearance. The fortress structures were restored as early as 1946 and are under state protection. This part of the fortress is a venue for cultural and sporting events.

The original idea of the Smederevo fortress was to build a fortified castle for the despot and his family on the border with the Kingdom of Hungary, where they could easily take refuge with their northern elder neighbor in case of danger from the Ottomans. Construction began in 1428 under the leadership of Thomas Kantakouzenos, and over the next half-century, the fortress was repeatedly expanded and modified to take its current shape by 1480. According to research, four distinct phases of Serbian construction and one phase of Ottoman construction can be identified.

The first phase of construction included the erection of a small town with internal buildings, mostly wooden. It was surrounded by low ramparts, water channels, and rivers, and had three gates: one facing the settlement and two facing the Danube.

After the construction of the Small Town, it became clear that it was insufficient, and expansion began immediately by adding long ramparts that would form the Large Town. The fact that only the Small Town was initially planned is evidenced by the cannon openings, which today face the ramparts built later. It is impossible to determine with certainty how far and to what extent the Large Town was built in 1439, the year the Ottomans besieged Smederevo, but it was undoubtedly in its final stage, as it seemed like a wonder of the world to the besiegers, although they had already encountered the Constantinople walls, which served as a model for Smederevo's. So, it was likely more or less completed, though it is unclear whether all the towers on the ramparts were fully finished. During this period, modifications were also made to the tops of some towers, which were vaulted to accommodate cannons that were used against the enemy in 1439.

After the restoration of the Serbian state by the Treaty of Szeged in 1444, reconstruction of the fort began, strengthening some towers to carry cannons and replacing wooden buildings in the small town with stone ones. During this period, the tower of the Nine Headed Brothers and the Seven Headed Brothers was added to the Small Town; it was lower than the others and served as a grain storage. Inside the Small Town, near the Danube wall, there was a reception hall where medieval court life took place. The Small Town also housed a jewelry workshop, a printing house, and a mint. Within the Large Town were the court and Annunciation churches; the latter housed the supposed relics of Saint Luke. The Large Town was also known as a craft and trade center, hosting a colony from Dubrovnik.

During the third phase of construction, which cannot be precisely dated due to continuous building, the ramparts on the Danube side with one tower and on the Jezava side with three towers were strengthened.

The fourth phase of construction included the addition of four towers on the Danube rampart facing the Kingdom of Hungary. It is believed this was done at the Sultan's request because Despot Đurađ considered the Hungarians his allies and thus did not need to fortify that side. However, there is a possibility that the despot did this on his own initiative to protect himself in case of an attack by the Hungarians due to disagreements between him and Hunyadi, which did occur. This may have been done after Hunyadi captured Đurađ following the Second Battle of Kosovo in 1448.

Based on their construction method, it can be concluded that these towers were built in peacetime, as unlike others built in haste, these four were made of red brick, resembling the red-gray Byzantine structures.

Smederevo fortress's debut in war participation on the side of the Turks was in 1480, shortly after the Battle of Golubac Field in 1476. The Ottomans used materials obtained from the demolition of the former Annunciation Church in the Large Town to build a low artillery tower at the mouth of the Jezava and Danube. Using materials from other sides, two more such towers were built at the vertices of the fort's triangular base, and finally, another low artillery tower protecting the city entrance tower, which disappeared due to an ammunition explosion in 1941.

In the small town was the court of Despot Đurađ, consisting of a residential residence and a reception palace. It was built between 1428-1430 as a specially fortified castle, surrounded by water on all sides. The small town has a donjon, a well in the center, and an entrance from the southwest through a system of double gates.

The court is separated from the Large Town, which has 19 towers, by a moat filled with water. The Small Town has 6 towers, one of which bears an inscription clearly showing that the city was built by Đurađ Branković. The towers are about 11 meters wide and about 20 meters high. They are made of rubble stone in good lime mortar. The wall thickness reaches 4.5 meters, with the thickest walls on the landward side of the fortress.

In the Large Town, there was a metropolitan complex with auxiliary rooms and buildings for the population. The walls are poorly preserved, as the building was later demolished and partially rebuilt. The choir wall and part of the northern wall have survived. Only the foundation remains of the western part of the wall.

The Small Town served Despot Đurađ as a fortified castle, from the only large hall of which four two-section windows on the city wall have survived to this day, while the expanses of the Large Town were necessary to accommodate the numerous garrison, which could number up to a thousand men, while the besieging forces numbered tens of thousands.

During World War I and the Austrians' crossing of the Danube, the Old Town was bombarded by 42-centimeter guns and was heavily damaged. In the autumn of 1938, one million dinars were allocated for restoration.

In World War I, the wall on the Danube side suffered the most damage, and at the beginning of World War II, German occupation forces used the Large Town to store about 400,000 tons of ammunition, equivalent to the explosive power of the atomic bomb dropped on Hiroshima. As a result of the explosion of this depot in June 1941, two and a half thousand people died. During World War II, the city of Smederevo received reparations from the Germans, the only such case in the world.

In recent years, conservation work has been carried out on the remains of the Annunciation Church found in the Large Town, built on the foundation of an old Byzantine church constructed in 1439, the same year the construction of the Smederevo fortress was completed. At that time, the church was the center of the city.

Sources:

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

https://otdihvserbii.ru/smederevska%D1%8F-krepost%D1%8C/