1.The Fortress on the Road to Constantinople
Rusokastro — "Rosokastron" to the Byzantine writers — was one of the fortresses in the hinterland of the Gulf of Burgas, together with Aytos, Develt, Skafida and Pyrgos (Burgas). Through it ran the road by which Byzantine armies advanced into the Bulgarian lands and by which the Bulgarians struck back: from Aytos to Rosokastron, Develt, Skopelos, Bryse, Vize and Constantinople. Because of this position it changed hands many times.
2.Between Two Empires (1323–1331)
When Tsar George II Terter died in the spring of 1323, Andronikos II was in Thrace. The news prompted the Bulgarian towns from Sliven to the sea — among them Rusokastro — to pass voluntarily, by agreement, under Byzantine rule, and the emperor appointed governors of his own choosing. Michael III Shishman soon recovered most of them, but the peace with Byzantium of 1328–1329 did not last.
After Michael's death at Velbazhd in July 1330, Andronikos III, set on avenging the expulsion of his sister Theodora from Tarnovo, attacked Bulgaria and with little effort took Anchialos, Mesembria, Aytos, Ktenia, Rusokastro and Yambol. The new tsar, Ivan Alexander, set out for south-eastern Thrace as soon as he took power in the spring of 1331 and with surprising speed recovered all these fortresses except Mesembria.
3.The Battle of Rusokastro
Andronikos III responded at once: he marched into the recovered lands and pitched his camp near Rusokastro, while Ivan Alexander was encamped near Aytos. Tatar mercenaries came to the Bulgarian tsar's aid — according to Kantakouzenos he had summoned them against his uncle Belaur, who had raised Vidin in revolt, but they arrived near Rusokastro and decided the outcome of the battle.
The battle of 18 July 1332 — Božilov and some other scholars date it a year earlier, to 18 July 1331 — ended in a Bulgarian victory. On the emperor's side, commanding the imperial detachment, was Manuel Asen, brother-in-law of the grand domestic John Kantakouzenos. A note in Ivan Alexander's Psalter praises the victory: the tsar mightily drove back the Greek emperor and took the fortified towns — Nesebar and the whole coast together with Romania.
4.The Peace
Ivan Alexander did not use his advantage to invade the empire: Belaur's revolt in the north-west and the changed situation in Serbia, where Stefan Dušan overthrew his father in September 1331, demanded his attention. The peace concluded near Rusokastro provided for a marriage between the Bulgarian tsar's firstborn son, Michael Asen, and Andronikos III's daughter. The wedding was postponed because the children were too young and took place in 1338 or 1339. Only fifteen days after the battle came news that the Turkish emir Umur Bey had raided near Gallipoli — a sign of the new danger that would soon overshadow the old rivalry between Bulgarians and Byzantines.