Which gulf lay along the north side of the ancient Thermopylae roadway?

The gulf along the north side of the ancient Thermopylae roadway was the Malian Gulf. In 480 BC, this body of water formed one side of the narrow coastal passage while steep hills formed the other, making Thermopylae a natural defensive position against the much larger Persian army.

The shoreline was much closer to the roadway in antiquity than it is today. Sediment carried into the gulf has gradually extended the land, leaving the modern battlefield several kilometres inland. Ancient descriptions also emphasize that parts of the route crossed marshy ground and constricted causeways rather than a uniformly narrow strip beside open water.

The Malian Gulf was strategically important because it connected the land battle with the Greek naval operation at Artemisium. The allied Greeks aimed to block the Persian army at Thermopylae while preventing the Persian fleet from bypassing the position by sea. The Pagasetic Gulf, Saronic Gulf, and Corinthian Gulf were elsewhere in central or southern Greece and were not the water beside the pass.