Complex sentence: By the Middle Ages Hartlepool was growing into an important (though still small) market town, one of the reasons for its escalating wealth being that its harbour was serving now as the official port of the County Palatine of Durham.
Split into simpler sentences: By the Middle Ages Hartlepool was growing into an important (though still small) market town. One of the reasons for its escalating wealth being that its harbour was serving now as the official port of the County Palatine of Durham.