From the 4th century, the Empire's Balkan territories, including Greece, suffered from the dislocation of barbarian invasions. The raids and devastation of the Goths and Huns in the 4th and 5th centuries and the Slavic invasion of Greece in the 7th century resulted in a dramatic collapse in imperial authority in the Greek peninsula. Following the Slavic invasion, the imperial government retained formal control of only the islands and coastal areas, particularly the densely populated walled cities such as Athens, Corinth and Thessalonica, while some mountainous areas in the interior held out on their own and continued to recognise imperial authority. Outside of these areas, a limited amount of Slavic settlement is generally thought to have occurred, although on a much smaller scale than previously thought. However, the view that Greece in late antiquity underwent a crisis of decline, fragmentation and depopulation is now considered outdated, as Greek cities show a high degree of institutional continuity and prosperity between the 4th and 6th centuries AD (and possibly later as well). In the early 6th century, Greece had approximately 80 cities according to the Synecdemus chronicle, and the period from the 4th to the 7th century AD is considered one of high prosperity not just in Greece but in the entire Eastern Mediterranean.