Wikipedia
Doriscus was an ancient settlement in Thrace (modern-day Bulgaria), on the northern shores of Aegean Sea. It was notable for remaining in Persian hands for many years after the Second Persian invasion of Greece, and remained thus known as the last Persian stronghold in Europe.
The ancient city was located in a vast plain of the same name, where the River Hebrus was crossing in the middle.
The city fell into the Persian hands without any resistance, during the Darius the Great' Scythian Expedition and inclusion of the Black Sea territories into the Persian Empire, around 522 BCE. He built a Royal Fortress and stationed a large number of Persian troops there.
Herodotus (7.59) reports that Xerxes the Great, during his campaign of invading the Greek City States in 480 BCE, was the first place he stopped to review his troops after crossing the Hellespont.
Herodotus also writes that Xerxes I of Persia made Mascames, son of Megadostes, governor of Doriscus in order to replace the man Darius I had appointed.
Doriscus is notable as it was one of the few Persian towns in the Balkans that remained under the Persian sway, even after the Second Persian invasion of Greece. The Athenian general Cimon that led the conquests after the Persian retreat was unable to capture it. Herodotus states that Doriscus "was never taken" from the Persians. Its governor Mascamos was honored by the Persian king for his defence.