Until the 17th century, the area was an island surrounded by a large area of fenland, a type of swamp. It was coveted as an area difficult to penetrate, and was controlled in the very early medieval period by the Gyrwas, an Anglo-Saxon tribe. Upon their marriage in 652, Tondbert, a prince of the Gyrwas, presented Æthelthryth (who became St. Æthelthryth), the daughter of King Anna of the East Angles, with the Isle of Ely. She afterwards founded a monastery at Ely, which was destroyed by Viking raiders in 870, but was rebuilt and became a famous Abbey and Shrine. The Fens were drained beginning in 1626 using a network of canals designed by Dutch experts. Many Fenlanders were opposed to the draining as it deprived some of them of their traditional livelihood. Acts of vandalism on dykes, ditches, and sluices were common, but the draining was complete by the end of the century.