The Forum, the very heart of the political, administrative and social life of the city, was a square surrounded by public buildings. It was first built as early as in the 2nd century BC.
• Arcades: the square took on the appearance still visible today in the first half of the 1st century AD. To the east and west, arcades were erected to house several shops; the courthouse used to stretch onto the southern end of the square. The look of the northern side is almost totally unknown, but for a round building provided with stairs, identified as the Comitium of Republican times, which accommodated the gatherings of the citizens.
• Columns: the columns of the eastern wing of the arcades have survived to date and were raised and integrated with bricks in the 1930s. Their composite capitals, as well their shafts and bases, are made of Aurisina limestone and date from the later period of the Antonine dynasty (about 170-180 AD).
• Square: 141 metres long and 55 metres wide, the square is entirely paved with Aurisina limestone slabs of same width and variable length. They date back to the time of the Julo-Claudian emperors (first half of the 1st century AD), but some of them were replaced over time with recycled materials, including some inscriptions.
• Cycle of Jupiter Ammon and Medusa: a balustrade of parallelepiped blocks used to run along the architrave of the arcades, just above the capitals, interspersed with slabs above the spaces between columns. The slabs were decorated in relief with putti or eagles holding wreaths, while the blocks held the heads of Jupiter Ammon and Medusa. The two motifs had been chosen to recall and reiterate the width of Roman dominions: from the west (Medusa) to the east (Jupiter Ammon, a deity cherished by Alexander the Great), that is, almost the entire world then known by man.
• Inscription of Titus Annius: the inscription carved on a base or altar represents a remarkable reference for the history of Aquileia in the 2nd century BC. It mentions Titus Annius (Luscus), a triumvir entrusted to lead the second contingent of colons who had reached the city in 169 BC, a figure already known from literary sources (Titus Livy). This inscription provides an important list of the magistrate’s duties: drafting laws for the colony’s administration, replacing the members of the local senate, and erecting a temple, whose precise whereabouts are unknown, although it must have looked onto the square.
• Courthouse and road: a long stretch of an urban road of Aquileia can still be seen to the south of the forum. It is named Decumanus of Aratria Galla from the name of the matron from Aquileia who munificently sponsored the paving of the road in the first half of the 1st century AD. The road bounds the site of the courthouse, which used to stand along the short side of the forum, only partially visible today. The large construction (90 x 29 m) serving judiciary and other purposes was built during the early Augustan era, but was entirely reconstructed in the late 2nd century AD, with the addition of two opposite apses. It used to face on the decumanus with two doors, while a monumental entrance to the opposite side used to connect it with the arcades of the forum. At an unknown time, most probably in the 6th century, powerful defensive walls were erected above the construction, by recycling several architectural and sculptural elements from its ruins.