The Belkıs Tombolo measures roughly 1,700 meters wide and 1,500 meters long. That figure — wider than it is long — captures something unusual about it: this is not a thin spit but a substantial platform of land, broad enough to carry two separate highways, one along the eastern shore toward Bandırma and one along the western shore toward Erdek.

The two shores have different characters. To the east, facing Bandırma Gulf, sand dunes have accumulated along the shoreline. To the west, facing Erdek Gulf, a sandy beach runs the length of the tombolo. Between them, at the midpoint of the isthmus, lies marshland — the kind of wet, reed-choked ground that forms where sediment piles up and drainage is poor. In geological terms, the tombolo is still young and dynamic, its margins shifting as currents and waves continue to deposit material. It is a landscape in process, not a finished thing.

Ancient sources are not entirely clear on when or how the tombolo formed. The Wikivoyage account of the area notes that the Kapıdağ Peninsula was one of the Marmara Islands until roughly 2,000 years ago — earth tremors may have lifted the bedrock, and the channel gradually silted until it became a sandbar and then an isthmus. The suggestion that Alexander the Great built a causeway across it, sometime after his conquest of the region in 334 BC, reflects how significant the crossing point was even then: a peninsula worth bridging for military and commercial reasons.

The peninsula's classical Greek name was Arctonnesus — the Bear Peninsula. When it was still an island, the city of Cyzicus stood on its southern shore facing the mainland, its harbor giving it command of the sea lanes between the Aegean and the Black Sea. The tombolo that formed beneath Cyzicus's feet changed the city's relationship with the water, making it a peninsula city rather than an island one. That transition was not the city's undoing — Cyzicus flourished for centuries afterward — but it altered the strategic geometry that had defined it.

Because the tombolo connects the peninsula to the mainland, it has always been the sole land approach to Cyzicus and later to Erdek. Any army wishing to besiege the peninsula had to either cross this narrow neck or transport troops by sea around it. In 73 BC, when the Pontic king Mithridates VI surrounded Cyzicus with a reported 300,000 soldiers, his engineers had to ship a portion of his forces across the water to fully invest the city — the tombolo alone was not wide enough to encircle it. The Roman general Lucullus used the same geography against him, positioning his army to cut Mithridates' supply lines through the isthmus while the city held out.

Six centuries before that, the Spartan admiral Mindarus held the peninsula during the Peloponnesian War, until the Athenian fleet under Alcibiades trapped him in 410 BC and annihilated his ships in the waters to the south. Geography made Cyzicus valuable; it also made the tombolo a chokepoint that shaped every conflict for control of the place.

Today the Belkıs Tombolo is mundane in the way that consequential places often become once the drama has passed. The two roads crossing it are ordinary regional highways. The eastern side's dunes provide some visual texture; the western beach draws summer visitors from Bandırma and beyond. The central marshland is a habitat for wading birds and a seasonal wetland that floods after winter rains. The site of ancient Cyzicus itself lies to the east of the tombolo, about 5 km from Erdek, protected by Turkey's Ministry of Culture but largely unexcavated — a sprawl of buried foundations and exposed stones in a landscape of fields and reeds.

From the tombolo, looking east on a clear day, you can see the low mounds that indicate where the ancient city's amphitheater once stood — its diameter nearly 500 feet, one of the largest in the Roman world. Looking west, Erdek's harbor is visible along the shore. The strip of land between them is the reason both places exist where they do.