Polish archaeologists in Sudan discovered functions of some of almost one hundred monumental defensive structures. They were built between 4th and 6th centuries AD. The structures originate from the time before and just after the Christianisation of pagan kingdoms in the mid-sixth century. Majority of the fortresses originally served as refuges. In times of peace, people lived in villages along the Nile, close to the fields. In several fortresses researchers also found traces of trebuchets, ballistic devices that tossed stones at distances up to 100 m.
Among the structures are ramparts that were several meters high and several meters wide. Depending on the place, they were made of stones or mud bricks, sometimes combining both techniques. All blocks were bonded with mud mortar. Walls covered an area of several hectares.
In Selib a rectangular walled structure with a church and a well in the middle were studied. The surrounding wall did not have corner towers, and was relatively thin, so the scientists doubted its defensive functions. Researchers discovered sixteen bands of stone stairs leading to the top of the wall, and gained confidence that the structure had a practical purpose. An easy way to climb to the top of the wall was needed to take defensive action. The wall itself turned out to be much higher than previously thought.
Defensive structures were built 20-30 km apart. Between them were observation towers that allowed early detection of impending danger. The 4th cent. AD was the time of the collapse of the Kingdom of Meroë, the collapse of the central government, regional divisions. All this in addition meant that the danger was real, and the borders were unguarded.
(after Nauka w Polsce & Bogdan Żurawski)