The Balkan provinces were denuded of troops , however .
The Balkan provinces were were devastated by invasions of Slavs and Bulgars ; to protect Constantinople and its vicinity against them , the emperor built the Anastasian Wall , extending from the Propontis to the Black Sea .
The Balkan provinces were denuded of troops , however , and were devastated by invasions of Slavs and Bulgars ; to protect Constantinople and its vicinity against them , the emperor built the Anastasian Wall , extending from the Propontis to the Black Sea .