Ruse is an ancient Bulgarian borough and a port on the Danube River. People settled here in ancient times since the river provided a living. As seen on the prehistoric settlement hill, the people in that earliest period lived in a place with clearly distinguished residential and public buildings, streets and neighbourhoods. The clay idols worshiped as guardians of life that have been found during excavations are now kept in the museum.

The castle was on the main road from where Belgrade is today to the delta of the Danube River. As the milestones that were once at the exits of Sexsaginta Prista indicate, that was the starting point of roads to the towns of Nove (Svishtov), Martianopolis (Devnya), Odesos (Varna) and Nicopolis ad Istrum (near the village of Nikyup, district Veliko Tarnovo).

Written sources show three versions of the name of the castle: Sexsaginta Prista, Sexsanta Prista or just Prista. The root of the first part of the name is the Latin numeral sexsaginta, which translates as “sixty”. The second part comes from Greek (prista, pristis) and means a kind of an ancient speedy battle ship with oars. It is supposed that there were 60 berths in Prista, hence the name of the castle: the port town for sixty ships.

Felix Kaniz, an Austrian-Hungarian scholar, was the first one who identified Ruse of today with the ancient castle of Sexsaginta Prista. The Skorpil brothers were the first ones to make archaeological excavations. They described the castle, investigated some burial places, collected coins and items used in the day-to-day life and published some of the inscriptions they had found.

No systematic excavations have been made on the site where the castle used to be. However, some rescue excavations have been made on spots jeopardised by modern city development. It is through them that the northeastern battle tower, a part of the northern wall and the remains of four buildings have been investigated. The tower is rectangle, its inner sizes 4,00 Х 3,80 m and its walls 2,70 m thick. 50 m of the northern wall are preserved. Its width is between 2,75 and 3,00 m.

An official inscription dated to the reign of Diocletianus (298-299) announces that the fort was restored as a praesidium (a large garrison settlement) after it was ruined by the raids of the Goths (250 A.D).

Sexsaginta Prista shared the doom of the rest of the forts on the right bank of the Lower Danube. It was destructed by Avars and Slavs in the late 6th and early 7th century. At a later time, in 9th-10th century, its ruins served as the foundations of a medieval Bulgarian borough called Ruse. In the course of time it became a fort on the Danube River and resumed its guarding functions. The archaeological finds dated to the First Bulgarian Kingdom provide evidences of the existence of that Bulgarian fort and settlement.

In the heathen period (till the mid 9th century) the settlement was called Ruse on the name of a feast called Roussalii. Researchers say that there was a cult to Maiden Roussa who was believed to protect young men in battle units.