Here is the text:
Shahrbaraz, along with a smaller army, later managed to slip through Heraclius' flanks and bee-lined for Chalcedon, the Sasanian base across the Bosphorus from Constantinople.
Please help me understand bolded text. If I'm not wrong, "bee-lined for Chalcedon" means "he went directly to Chalcedon".
"to slip through" I think means "to runaway from the inside of enemy's army"
But the hardest part is "the Sasanian base across the Bosphorus from Constantinople". I have no idea about this.
Here is the source: https://en.wikipedia.org/wiki/Shahrbaraz#Siege_of_Constantinople