Complex sentence: The site lies close to the present borders of Yorkshire, Nottinghamshire and Lincolnshire; in Aethelfrith's time this area lay on the southern reaches of Northumbria, a dangerous marshy region close to the border with Lindsey and easily accessible from the East Anglian kingdom.
Split into simpler sentences: The site lies close to the present borders of Yorkshire, Nottinghamshire and Lincolnshire. In Aethelfrith's time this area lay on the southern reaches of Northumbria, a dangerous marshy region close to the border with Lindsey and easily accessible from the East Anglian kingdom.