Complex sentence: Certain of their fate, some swam to nearby Dursey Island, where they were murdered or captured in the water; the remaining defenders repelled another assault and sealed themselves in the cellar of the castle.
Split into simpler sentences: Certain of their fate, some of the surviving defenders attempted to swim to nearby Dursey Island where they were murdered or captured in the water. The rest of the surviving defenders, after repelling another assault, sealed themselves in the cellar of the castle.