Complex sentence: The Danish attacks had been particularly damaging to the monasteries, and though Alfred founded monasteries at Athelney and Shaftesbury, these were the first new monastic houses in Wessex since the beginning of the eighth century.
Split into simpler sentences: The Danish attacks had been particularly damaging to the monasteries. In response Alfred founded monasteries at Athelney and Shaftesbury; the first new monastic houses in Wessex since the beginning of the eighth century.