Complex sentence: The earliest, and therefore probably most reliable, account is that of the medical writer Thessalos, who in the fifth century BC wrote that the attackers discovered a secret water pipe leading into the city after it was broken by a horse's hoof.
Split into simpler sentences: The earliest, and therefore probably most reliable, account is that of the medical writer Thessalos. He wrote, in the fifth century BC, that the attackers discovered a secret water-pipe leading into the city after it was broken by a horse's hoof.