The Roman and Roman-Byzantine fortress of Carsium was built over a Getae fortification, starting from the second half of the first century AD. In the early 2nd century AD, during the Dacian wars, in 103 AD, Emperor Trajan reinforced the fortress with stone walls.

"During the Emperor Nerva Trajan Augustus, conqueror of the Germans and the Dacians, the Supreme Pontiff, invested with tribune power the seventh time, hailed imperator for the fourth time, consul for the fifth time, the father of our country, under Q. Fabius Postuminus linked imperial praetorian rank, ala II Hispanorum (built this camp)”

In reconstitution of the inscription, Vasile Pârvan writes Ala II Hispanorum because this name appeared on a military milestone discovered at Hârșova dating from the year 200. Most likely, the military unit for which the fortification was built was Ala (Gallorum) Flaviana, as attested by recent discoveries, at the very end of the 1st century and the beginning of the 2nd century AD.

The fortress was defended during antiquity by the soldiers of Legio I Italica, by the salesmen of Classis Flavia Moesia, the milites scythicii or soldiers of Legio I Iovia Scythia. Their mission was to defend the crossing ford near the Danube, one of the most important on this segment of the limes.

The fortress is mentioned frequently in ancient documents from the 2nd century AD until the 7th century (Ptolemy, Tabula Peutingeriana, Itinerarium Antonini, Notitia Dignitatum, Hierocles, Procopius, Geographer of Ravenna) named Carsum, Carsio, Carso, Cars. Most likely, the toponym is of Thracian origin and is related to the appearance of the stony ground.

Historical sources indicate repeated destruction and reconstruction of the city under Emperors Constantine the Great and Justinian. Early but also recent research uncovered archaeological materials of great value, both scientific and for the museum, in the fortification and in its necropolises.

The largest number of milestone columns (Roman road blocks) in Dobrogea has been recovered from Hârșova: ten so far. This demonstrates that during ancient times the authorities have permanently repaired and maintained in optimum conditions the roads that assured the connection, either along the Danubian limes or other settlements within the province. Hence, the ancient towns importance as a trade and transit area, as is demonstrated by the exceptional discoveries made over time.

Both from the systematic excavations and from fortuitous discoveries, archaeological materials that show not only trade links with the largest centres in the Roman Empire, but also production activity, have been unearthed. The size and importance of the ancient locality suggests that there were ceramic workshops, quarries for the extraction of limestone, carving workshops for the production of glass vessels, metallurgical workshops, wood processing, leather, etc. All this demonstrates that the crafts palette attested in Carsium is diverse and varied. This fact shows a thriving settlement, defended by strong walls of stone, with the harbour at the Danube, with imposing buildings of marble or limestone. Several buildings were highlighted. Between them stand a Christian basilica, baths, and certain characteristic elements of fortification architecture.

Archaeological discoveries show that there was a strong spiritual life. Certainly, many come from temples built here, dedicated to the Roman deity of the time. Others belonged to investigated tombs in the ancient necropolis. One of the most important dimensions of the Carsium civilization is Christian. During recent researches an apse construction in the fortress was identified, most likely a Christian basilica. From older excavations come vessels with Christian symbols (fish, crosses), ceramic oil lamps with a cross-shaped handle. Several elements of the Christian ritual have been identified in the graves. All this is explicable given the fact that, during the 5th century, Carsium was numbered among the 14 cities that were residences of the bishops from Scythia.

After the end of the Roman-Byzantine rule at the Lower Danube, the history of the medieval fortress from Hârșova begins. During the 10th century, it is rebuilt by the Byzantines and later by the Genoese. Between the 15th and the first part of the 19th century, it is ruled by the Turks. According to chronicler Evlia Celebi, the city was mentioned in Ottoman documents by the name Harisova. The medieval fortress is better known during this period owing to historical documents and the fact that traces of the inner wall defending the city, for an area of about 24 hectares, are seen in several places today. The Commander Tower, on the northern side of the small enclosure, is preserved at the height of over 9 m. At the Danube, a monumental wall, about 40 m long, marks the area where the ancient harbour served until the destruction of the city, for 17-18 centuries!

During the last period of its existence, the city was the battlefield of bloody clashes between the Ottoman and Tsarist Empire and has been subject to much destruction. By the Peace of Adrianople, the Sublime Porte is forced to demolish the fortifications on the right bank of the Danube, and the fortress is blown up. Modern Hârșova is built in its place.