AQUILEIA, an ancient town of Italy, at the head of the Adriatic at the edge of the lagoons, about 6 m. from the sea, on the river Natiso (mod. Natisone), the course of which has changed somewhat since Roman times. It was founded by the Romans in 181 B.C. as a frontier fortress on the north-east, not far from the site where, two years before, Gaulish invaders had attempted to settle. The colony was led by two men of consular and one of praetorian rank, and 3000 pedites formed the bulk of the settlers. It was probably connected by road with Bononia in 175 B.C.; and subsequently with Genua in 148 B.C. by the Via Postumia, which ran through Cremona, Bedriacum and Altinum, joining the first-mentioned road at Concordia, while the construction of the Via Popilia from Ariminum to Ad Portum near Altinum in 132 B.C. improved the communications still further. In 169 B.C., 1500 more families were settled there as a reinforcement to the garrison. The discovery of the goldfields near the modern Klagenfurt in 150 B.C. (Strabo iv. 208) brought it into notice, and it soon became a place of importance, not only owing to its strategic position, but as a centre of trade, especially in agricultural products. It also had, in later times at least, considerable brickfields. It was originally a Latin colony, but became a municipium probably in 90 B.C. The customs boundary of Italy was close by in Cicero's day. It was plundered by the Iapydes under Augustus, but, in the period of peace which followed, was able to develop its resources. Augustus visited it during the Pannonian wars in 12-10 B.C. and it was the birthplace of Tiberius's son by Julia, in the latter year. It was the starting-point of several important roads leading to the north-eastern portion of the empire - the road (Via Iulia Augusta) by Iulium Carnicum to Veldidena (mod. Wilten, near Innsbruck), from which branched off the road into Noricum, leading by Virunum (Klagenfurt) to Lauricum (Lorch) on the Danube, the road into Pannonia, leading to Emona (Laibach)  and Sirmium (Mitrowitz), the road to Tarsatica (near Fiume) and Siscia (Sissek), and that to Tergeste (Trieste) and the Istrian coast.
In the war against the Marcomanni in A.D. 167, the town was hard pressed; the fortifications had fallen into disrepair during the long peace. In A.D. 238, when the town took the side of the senate against the emperor Maximinus, they were hastily restored, and proved of sufficient strength to resist for several months, until Maximinus himself was assassinated. The 4th century marks, however, the greatest importance of Aquileia; it became a naval station and, probably, the seat of the corrector Venetiarum et Histriae; a mint was established here, the coins of which are very numerous, and the bishop obtained the rank of patriarch. An imperial palace was constructed here, in which the emperors after the time of Diocletian frequently resided; and the city often played a part in the struggles between the rulers of the 4th century. At the end of the century, Ausonius enumerated it as the ninth among the great cities of the world, placing Rome, Mediolanum and Capua before it, and called it "moenibus et portu celeberrima." In A.D. 452, however, it was destroyed by Attila, though it continued to exist until the Lombard invasion of A.D. 568. After this the patriarchate was transferred to Grado. In 606 the diocese was divided into two parts, and the patriarchate of Aquileia, protected by the Lombards, was revived, that of Grado being protected by the exarch of Ravenna and later by the doges of Venice. In 1027 and 1044 Patriarch Poppo of Aquileia entered and sacked Grado, and, though the pope reconfirmed the patriarch of the latter in his dignities, the town never recovered, though it continued to be the seat of the patriarchate until its formal transference to Venice in 1450. The seat of the patriarchate of Aquileia had been transferred to Udine in 1238, but returned in 1420 when Venice annexed the territory of Udine. It was finally suppressed in 1751, and the sees of Udine and Gorizia (Görz) established in its stead. Its buildings served as stone quarries for centuries, and no edifices of the Roman period remain above ground. Excavations have revealed one street and the north-west angle of the town walls, while the local museum contains over 2000 inscriptions, besides statues and other antiquities. The cathedral, a flat-roofed basilica, was erected by Patriarch Poppo in 1031 on the site of an earlier church, and rebuilt about 1379 in the Gothic style by Patriarch Marquad. The narthex and baptistery belong to an earlier period. Of the palace of the patriarchs only two isolated columns remain standing. The modern village (pop. 2300) is rendered unhealthy by rice-fields.
See T.W. Jackson, Dalmatia, Istria and the Quarnero (Oxford, 1887), iii. 377 seq.; H. Maionica, Aquileia zur Romerzeit (Görz, 1881), Fundkarte van Aquileia (Görz, 1893), "Inschriften in Grado" (Roman inscriptions removed thither from Aquileia) in Jahreshefte des Osterr. Arch. Instituts, i. (1898), Beiblatt, 83, 125.
 This road is described in detail by O. Cuntz in Jahreshefte des Osterr. Arch. Inst. v. (1902), Beiblatt, pp. 139 seq.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)