Complex sentence: The walls became unnecesary as the city grew further and became well protected by the ever expanding military strength of the Republic and of the later Empire, and the city was essentially unwalled for the first three centuries of the Empire.
Split into simpler sentences: The walls became unnecessary as Rome became well protected by the ever expanding military strength of the Republic and of the later Empire. As the city continued to grow and prosper, it was essentially unwalled for the first three centuries of the Empire.