Complex sentence: At the heart of the English economy prior to the Norman invasion was a mature agricultural system of villages comprising demesne lands supported by an open field system, with peasants usually holding their land in return for agricultural labour for their local lord.
Split into simpler sentences: At the heart of the English economy prior to the Norman invasion was a mature agricultural system of villages comprising demesne lands. This was a system of open fields with peasants usually holding their land in return for agricultural labour for their local lord.