An ancient Swedish fort is beginning to relinquish a dark secret it has held for 1500 years – that its walls bore witness to a brutal massacre. Although it’s unclear exactly what motivated the apparently merciless attack, it was probably linked to the chaos that swept across northern Europe following the collapse of the Western Roman Empire.

Sandby borg on the island of Öland was an impressive ringfort in the late 5 ^th century. Its walls stood 5 metres tall and enclosed a …