Complex sentence: During 868 they marched deep into Mercia and overwintered in Nottingham, where they were besieged by a joint force from the kingdoms of Mercia and Wessex.
Split into simpler sentences: During 868 they marched deep into Mercia and overwintered in Nottingham. At Nottingham they were besieged by a joint force from the kingdoms of Mercia and Wessex.