Thrace in late antiquity was a region bordered by the Balkan Mountains, the Black Sea, the Sea of Marmara, and the Nestos River. In the 4th-7th century, the term designated the traditional Thracian territory, the province of Thracia, and the diocese of Thraciae, embracing the provinces of Europa, Thracia, Haemimontus, Rhodope, Moesia II, and Scythia. The five major cities in Thrace proper were listed as Philippopolis, Augusta Trajana, Diokletianopolis, Sebastopolis, and Diospolis. The supreme military commander in the diocese of Thrace was the magister militum for Thrace. In the 6th century, after the construction of the Long Wall in Thrace to protect Constantinople from barbarian invasions, the office of the vicarius of the Long Wall was created. In the 4th through 7th century, the diocese of Thrace was invaded by Goths, Huns, Slays, and other peoples; finally the Slays and Bulgars settled in the area, almost all the cities were deserted, and the Thracian population retreated to the mountains. The metropolitan see of Thrace was located in Philippopolis. By the end of the 7th century, the administration of Thrace changed, though the details are unclear. From the 11th century, Thrace as an administrative unit usually appears combined with Macedonia under the command of the same strategos. Thrace seems to have later disappeared from official administrative nomenclature, but the term was broadly used by some antiquarian writers such as Kantakouzenos and Kritoboulos.