Complex sentence: They managed to breach the town's walls and massacred about 20,000 inhabitants which was pretty usual from their previous raids but this time, as they could not find enough food, they resorted to cannibalism.
Split into simpler sentences: They managed to breach the town's walls and massacred about 20,000 inhabitants, as they often did when they captured a town. However, this time, as they could not find enough food, they resorted to cannibalism.