When you visit Belgrade, Serbia you will probably see this octagonal tower of the Belgrade Fortress. It is the only medieval tower of the Belgrade Fortress that survived unitl today. In the time it was built it served as the mayor defensive tower of the fortress. Later it was used as a dungeon and in 2010 it was adapted into the museum. Fortunately the tower is well preserved and is a real eyecatcher today.
Wikipedia has a lot more information about this building. Part of it you will find below:
"History
The name of the tower, nebojša, means "fearless" in Serbian. In the medieval Serbia it was a common name of the tower which was most forward from the fortress. The tower was originally called the "White tower" and is mentioned under this name in 1572. Later, it was named "Timi?oara Tower" and after the original Nebojša Tower was destroyed in 1690, the name was transferred to the present tower.
The present Nebojša Tower was built c.1460 by the Hungarians, who ruled Belgrade at the time. When it was built, it was located at the entrance into the city's "Danube port", in the Lower Town of the fortress and was part of the defense system which was protecting Belgrade from the Ottoman invasion after the Siege of Belgrade in 1456. During the 1521 Siege of Belgrade, cannons from the tower successfully defended the city from July to August, until the Ottomans managed to set the tower on fire. Only then, they managed to conquer the city. The sultan Suleiman the Magnificent ordered for the tower to be rebuilt.
In the 18th century, the Nebojša Tower was also reconstructed. The old, upper part of the tower was lowered and a new floor with the vault was built. Old embrasures were walled up and the new, larger ones were opened to fit the modern, bigger cannons. At that time, the inner, war port, which was protected by the tower, still existed. In c.1725 Austrians built an arsenal at the base of the tower.
After regaining Belgrade in 1739, the Ottomans closed and covered the port with earth so the tower lost its military importance and was turned into the dungeon. Due to the terrible conditions and the practice of strangling the prisoners and throwing them through the openings of the tower into the river, the Nebojša Tower became one of Belgrade’s darkest symbols.
Hugo Buli (1875-1942), son of the noted Belgrade merchant Edidija Buli, was a student in Germany when he brought the first leather football to Serbia. Hugo, who was already playing football for "Germania" club in Berlin, managed to form the first football section within the "Soko" gymnastics society on 12 May 1896. On the field in front of the Nebojša Tower, the first football match in Serbia was played on 19 May 1896. The field wasn't marked with lines and had no goals, only bricks which marked where the goal should be. The players didn't know the rules, so Hugo was explaining everything. There were 50 spectators. New reports of the "gymnastics game" under the tower survived, though the newspapers referred to it as a fun, rather than as a proper sport. Hugo founded the first football association in Serbia in 1899, and is considered a father of football in Serbia.
Former armory was buried and effectively cut in two when the modern Donjogradski boulevard (today Vojvoda Bojovic boulevard) was built over it in 1935.
Architecture
The Nebojša Tower belongs to the oldest type of the early artillery cannon towers. When built, it represented an important achievement in the architecture of the time. It has a regular octagon base, with 2.8 m (9 ft 2 in) thick walls. It has a ground floor and three storeys, with the total height of 19 m (62 ft). All floors have embrasures, or the cannon openings, on all sides, so that invaders can be shelled whether they advance from the river or from the land. The tower used to have a leveled top with the battlements and protruded wooden corbels. When Austrians reconstructed the tower in the first half of the 18th century, the top with the battlements was replaced with the roof construction which remained to this day."
Source and further information: en.wikipedia.org/wiki/Nebojša_Tower/