One of the few city states who managed to maintain full independence from the control of any Hellenistic kingdom was Rhodes. With a skilled navy to protect its trade fleets from pirates and an ideal strategic position covering the routes from the east into the Aegean, Rhodes prospered during the Hellenistic period. It became a center of culture and commerce, its coins were widely circulated and its philosophical schools became one of the best in the mediterranean. After holding out for one year under siege by Demetrius Poliorcetes (304-305 BCE), the Rhodians built the Colossus of Rhodes to commemorate their victory. They retained their independence by the maintenance of a powerful navy, by maintaining a carefully neutral posture and acting to preserve the balance of power between the major Hellenistic kingdoms.