Complex sentence: Gainsborough was one of the capital cities of Mercia during the Anglo-Saxon period which had preceded Danish rule: it is understandable that the Viking kings would have been drawn to it as an administrative centre.
Split into simpler sentences: Gainsborough was one of the capital cities of Mercia during the Anglo-Saxon period which had preceded Danish rule. It is understandable that the Viking kings would have been drawn to it as an administrative centre.