The passage under the Arch leads into the inner square, the Tarsatica Principia forum. Today, at the foot of the rear of the St. Fabian and Sebastian church, there is still a well-preserved masonry, built using the ancient Roman technique opus mixtum, i.e. combining layers of carved blocks of local grey limestone with layers of thinner bricks. The partially preserved forum paving, as well as the remains of the peripheral rooms, whose walls were plastered and even partially painted in frescoes, were supplemented through conservation and restoration works. The decorated surfaces of the forum basilica that ran along the northern rim of the inner courtyard of our Principia, were quite prestigious. The layout of the Tarsatica military headquarters is a square with sides of about forty meters in length. It ascended in cascades adjusting to the slope of the terrain. On top of these remains, continuing on the already existing closed structure and using the available building materials, the people of the medieval, Renaissance, and Baroque Rijeka built their houses and thus made it a distinctive monument where the remains of the Late Antiquity period mix with architectural layers of subsequent periods. In the row of houses with mainly Baroque architectural features, which encloses the southern area of the St. Michael Square, above the remains of the Principia, the remains of Late Antiquity masonry walls have on occasion been preserved to a height of the second floor of the existing houses.
Numerous numismatic and other mobile findings confirm the existence and the continuity of life of the Tarsatica Principia from mid third century to mid fourth century AD. Consistent with a garrison facility of the Late Antiquity period, there are findings of rough and find pottery, glass vessels, parts of weaponry, armours, arrows, spears, horse paraphernalia, pens, jewellery, and bronze objects that, hypothetically, may be linked to the imperial cult, but also to eastern cults, then quite common among the military and, of course, to early Christianity. Other mobile findings, such as a Renaissance maiolica, testify to the early urban history of the modern city of Rijeka. The area of the inner courtyard has been treated by the conservationists and is exhibited as the Archeological Park – the Tarsatica Principia.
At the site of the new corner building rising at the intersection of Užarska and Medulićeva streets, i.e. former Decumanus Maximus and Cardo Maximus of ancient Rijeka (Tarsatica), the remains of an atrium and mosaic flooring of one city palace were found. Most certainly, it was one of the most luxurious residential buildings of the Roman Tarsatica. Upon research, the mosaic was relocated to the adjacent building of the head office of the Conservation Department in 26 Užarska Street, and by appointment, it can be seen at the Gallery of the Conservation Department in Rijeka. There too, during archaeological research, floor mosaic tiles, foundations of a masonry wall and drainage canals were found. Therefore, we may conclude that the location of this Baroque palace, built through combining and upgrading a series of Gothic houses, was an area with luxurious residential buildings during Roman times as well.