Complex sentence: The Romans designed the testudo formation, which was formidable against armies consisting solely of mounted archers, reason for which the Parthians and later on the Sassanid Persians would employ cataphracts, which in combination with mounted archers caused the Roman legions to lose many important battles.
Split into simpler sentences: The Romans designed the testudo formation, which was formidable against armies consisting solely of mounted archers. In response, the Parthians, and later on the Sassanid Persians, would employ cataphracts, which in combination with mounted archers caused the Roman legions to lose many important battles.