Today all that is visible above ground in Burnum are two arcades and part of the arch of the larger, once central arcade, which belonged to the basilica (courthouse, council house)
The main structure in the military camp of Burnum was the principium (Lat. principium), or the command centre/headquarters. In Roman military camps, the Principium was built at the junction of the two main streets (the viae principalis and viae praetoriae), and the building was a monumentral structure designed as the main square (forum militare) of the military camp. Forums were most often surrounded by a peristyle, and altars to offer sacrifices were often placed in the centre of the courtyard. The peristyle was surrounded by a series of rooms that were not for everyday use, rather exclusively for religious and administrative purposes. They consisted of officers’ offices, archives, armouries and rooms for commanders’ meetings. The chambers of the commander (praetorium) were separate, they were always located near the pricipium, and most often were located in a building that imitated the appearance of an Italian villa with a courtyard and a peristyle. In the late Empire, it was common for the courtyard of the principium to have a transverse hall (basilica principiorum) that closed off one side of the forum. An integral part of the basilica was a raised platform for a speaker, the tribunal, from which commanders or senior officers could address the soldiers. The sanctuary (aedes or sacellum) could be reached from the courtyard or basilica, and it was usually a room with an apse. The military insignia (signa) and the legionary eagle were placed and venerated in that area. Additionally, it was common practice for the sanctuary to further serve as a place of worship of Jupiter or the imperial cult. The sanctuary also served as a treasury, where the soldiers’ salaries were kept, located in its underground (basement) part.
Data about the first investigations in the area of the principium were offered by Theodor Mommsen in 1859. In his notes, he provided a transcript of the report on the excavations conducted by the “curator” Đ. Sundečić by order of the Municipality of Kistanje. The next excavations in the area of the principium were led in 1912 and 1913 by E. Reisch for the Austrian Archaeological Institute in Vienna. Sixty some years later, M. Kandler, an employee of that institute, carried out minor revisory excavations in the same area within the principium of a structural complex that in reports and the scientific literature was incorrectly called the praetorium (commander’s residence).
The principium in Burnum is a building measuring 46 x 53 m, completed no later than the beginning of the reign of the Emperor Tiberius. After the unsuccessful rebellion of Scribonius against the Emperor Claudius, all of Burnum, including the legionary camp, experienced major urban and architectural changes. The approximate dimensions of the new building of the principium measured 104 x 73 m. It was composed of two elongated wings (with legionary offices) that flanked the interior courtyard, separated by arched arcades from the rear, shorter wing, which also contained the legionary shrine in which military trophies and battle insignia were kept.
After the departure of the army and the elevation of Burnum to the rank of a municipal settlement, the principium was transformed into a civil forum with adjacent facilities. Such a possibility suggests the existence of civilian contents in the principium of the castrum and further construction activity after the departure of the legion.
Today all that is visible above ground in Burnum are two arcades and part of the arch of the larger, once central arcade, which belonged to the basilica (courthouse, council house) built, in all probability, in the Trajan-Hadrian period, when Burnum had already acquired the status of a municipium. The basilica was placed transversally in relation to the longer axis of the principium, so it can be assumed that it fitted into the existing arcades as an entrance on the lengthy lateral wall towards the forum.
At the same time, the worship of the imperial cult was introduced in the area of the former principium, as shown by the finds of inscribed dedications to the emperors Trajan and Hadrian. During the restoration of the Metroac cult carried out by Antoninus Pius, the capitolium was also reconstructed, and on its front, on the edges of the façade, aediculae of the Metroac and Venus cults were added.
