Complex sentence: In the Anglo-Saxon period, the Humber was a major boundary, separating Northumbria from the southern kingdom, though at its height Northumbria did indeed cover areas south of the Humber.
Split into simpler sentences: In the Anglo-Saxon period, the Humber was a major boundary, separating Northumbria from the southern kingdoms. Though at it's height Northumbria did indeed cover areas south of the Humber.