Halmyris (Ancient Greek: Ἁλμυρίς) [1] was a Roman and Byzantine fort, settlement and naval port, located 2.5 km east of the village of Murighiol at the mouth of the Danube Delta in Romania.

[4] Halmyris served as a depot for supplies, colonisation and cultural exchange in the region for 1,100 years from the Iron Age to the Byzantine period.

Halmyris was built strategically on a rocky peninsula on the bank of the Danube River near the Black Sea and surrounded by marshes.

The region was inhabited during the Second Iron Age by the Getae or Dacians as is evident by the discovery of several cremation burials within a possible necropolis that dates to the 4th-2nd centuries BC.

The new layout of the fort walls consisted of an irregular polygon bolstered by 15 towers and at least two well-defended gateways in the north and the west.

[8] During the late Roman period two units of the military fleet, Classis in Plateypegiis and Musculi Schytici (which had shallow-draft ships, suited for the Danube Delta) may have been hosted by this city.

A series of earthquakes in the 4th century and later damaged the city and the course of the Danube changed leading to the silting up of Halmyris' harbour which decreased its economic and strategic importance.

In the 280's during the general reconstruction initiated by Aurelian (r.270-275) or Probus (r.276-282) it was completely dismantled and two monumental U-shaped towers were built flanking a 6.15 m wide vaulted gateway using stone from the previous phase and also votive altars and tombstones from the early necropolis.

It was initially a rectangular central aisled (nave) building with an entrance from the west, a forecourt (atrium) and a semicircular space roofed with a half-dome, exedra or apse.

In the third major phase the apse was enclosed into a rectangular building parallel to the N-S oriented street, the cardo maximus, leaving a 4.0-4.5 m wide ambulatorium behind the altar.