Complex sentence: The ruins of the castle and its turrets have become overrun by creatures such as giant rats, vampire bats and carnivorous vines, and its basement has become a den of thieves, pirates and other ne'er-do-wells.
Split into simpler sentences: The ruins of the castle and its turrets have become overrun by creatures such as giant rats, vampire bats and carnivorous vines. Its basement has become a den of thieves, pirates and other ne'er-do-wells.