The original military camp was probably occupied by the soldiers from the Legio VIII Augusta from 46 AD to 69 AD. 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 Belgrade Fortress. Construction began at the turn of the 2st 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. Connecting this way Via Militaris with the western parts of the empire, Singidunum became a major crossroad, not only for the local provinces (Moesia, Dacia, Pannonia, Dalmatia). The 6,000-strong legion became a major military asset against the continuous threat of the Dacians just across the Danube. Another step the Romans took to help strengthen Singidunum was the settlement of its legion veterans next to the fortress. In time, a large settlement grew out from around the "castrum". The main axis of urban development was along the modern Knez Mihailova Street, which was the main route of communication ("via cardo").