between 378 AD and 441 the Roman camp was repeatedly destroyed in the invasions by the Goths and the Huns. Legend says that Attila's grave lies at the confluence of the Sava and the Danube (under the fortress). In 476 Belgrade again became the border between the empires: the Western Roman Empire and Eastern Roman Empire (Byzantine Empire), and the Slav-Avar State in the north. The Celtic fortification was a primitive one, located on top of Terazije ridge, above the confluence of the Sava into the Danube, where the fortress still stands today. Celts also lived in small, open and fortified settlements around the fort, called opidums. Since it is not known for sure where the Celtic fort was; some historians suggest that it was rather close to the necropolises in Karaburma and Rospi Ćuprija. Celtic settlements belonged to the La Tène culture. The original military camp was probably occupied by the soldiers from the Legio VIII Augusta from 46 AD to 69. Early Singidunum reached its height with the arrival of Legio IV Flavia Felix which was transferred to the city in 86 AD and remained there until the mid 5th century. The presence of Legio IV prompted the construction of a square-shaped castrum (fort), which occupied Upper Town of today's fortress. Construction began at the turn of the 2nd century AD as since the early 100s, Legio IV Flavia Felix became permanently stationed in Singidunum. At first, the fortress was set up as earthen bulwarks and wooden palisades, but soon after, it was fortified with stone as the first stone fort in Belgrade's history. The remains can be seen today near the northeastern corner of the acropolis. The legion also constructed a pontoon bridge over the Sava, connecting Singidunum with Taurunum. Rectangular castrum covered what is today the Upper Town and the Kalemegdan Park. The castrum had tall walls, built from the white Tašmajdan limestone and spread over the area of to , being shaped as an irregular rectangle (approximately ). Middle Ages The Byzantine Emperor Justinian I rebuilt the fortress around 535. In the following centuries the fortress suffered continuous destruction under the Avar sieges. The Slavs (Serbs) and Avars had their "state union" north of Belgrade with the Serbs and other Slavic tribes finally settling in the Belgrade area as well as the regions west and south of Belgrade in the beginning of the 7th century.