The area around Mosonmagyaróvár has been inhabited since the 5th millennium BC, but settlement of the city proper can only be traced to around the 1st century, which was when the Roman Empire was extended to the Danube, creating the province of Pannonia. The Romans established a camp called Ad Flexum at the site of Mosonmagyaróvár; it is likely that the Hungarians from the Árpád era would name the place Óvár due to the Roman ruins which would still be present during the 11th century. The purpose of Ad Flexum was to defend the Mosoni-Duna, but the security the legions provided also drew civilian settlement, especially since a major east-west trade route ran through the area. Circa 170 AD, Germanic barbarians who lived north of the Danube river attacked the settlement, nearly completely destroying it. The Romans reconquered the area around the 3rd century, and the town once again prospered, likely with a population of three or four thousand people. After the Emperor Valentinianus died in 375, Hunnic invasions drove the populace away.
After the Honfoglalás, King Stephen ordered the building of a castle at Moson to defend the border. Settlers flocked around the wooden and then stone castle, and by the 11th century it was described as a strong fortress and bustling merchant town; by this time, it was also the county seat of Moson. However, in 1030, the Holy Roman Emperor Conrad II was able to conquer the castle on his way to the Rába. During the Crusades, Kálmán, King of Győr and Moson, was able to defeat a Swabian-Bavarian army of 15,000 men from the castle. There was significant industrial and urban development during the 13th century, when Moson once again found itself along a trade route. Mills and churches were built during this time. All advances were destroyed, however, by Ottokar II, a Czech king, when he leveled the castle at Moson in 1271.
Béla IV, King of Hungary at the time, did not consider it worthwhile to try and rebuild the
