of the heptarchy), which later merged with Mercia and then with Wessex. The influence of the early English settlers can be seen in the many place names ending in "-ham", "-ingham" and "-ton". Endings such as "-by" and "-thorpe" are also fairly common, indicating Danish toponyms: in the 9th century the region again came under attack, this time from Danes who killed the king, Edmund the Martyr. Several place names around the Fenland area contain Celtic elements; this has been taken by some scholars to represent a possibly significant concentration of Britons in the area. In the centuries before the Norman Conquest the wetlands of the east of the county began to be converted to farmland, and settlements grew in these areas. Migration into East Anglia must have been high: by the time of the Domesday Book survey it was one of the most densely populated parts of the British Isles. During the high and late Middle Ages the county developed arable agriculture and woollen industries. Norfolk's prosperity at that time is evident from the county's large number of medieval churches: out of an original total of over one thousand some 659 have survived, more than in any other county in Britain and the greatest concentration in the world. The economy was in decline by the time of the Black Death, which dramatically reduced the population in 1349. Kett's Rebellion occurred in Norfolk during the reign of Edward VI, largely in response to the enclosure of land by landlords, leaving peasants with nowhere to graze their animals, and to the general abuses of power by the nobility. It was led by Robert Kett, a yeoman farmer, who was joined by recruits from Norwich and the surrounding countryside. His group numbered some 16,000 by the time the rebels stormed Norwich on 29 July 1549 and took the city. Kett's rebellion ended on 27 August when the rebels were defeated by an army under the leadership of John Dudley, 1st Duke of Northumberland at the Battle of Dussindale. Some 3,000 rebels were killed. Kett was captured, held in the Tower of London, tried for treason, and hanged from the walls of Norwich Castle. By the late 16th century Norwich had grown to become the second-largest city in England, but over one-third of its population died in the plague epidemic of 1579, and in 1665 the Great Plague again killed around one-third of