Final answer:
The people of ancient Greece built Cyclopean walls and citadels, including the fortified Acropolis, and invested in a strong navy with triremes to protect against robbers and invaders.
Step-by-step explanation:
To protect themselves from invaders and robbers, the people of ancient Greece constructed formidable structures and employed strategic defense techniques. The Mycenaean Greeks, concerned with defense, built Cyclopean walls surrounding their citadels and the Acropolis. These walls were made up of large, roughly cut ashlar blocks, so called because the ancient Greeks believed that only the mythical Cyclopes could have moved such massive stones.
Another significant venture was the Athenian investment in a powerful navy, initiated by the general Themistocles. Using the silver from a discovered mine, Athens built a fleet of warships known as triremes. This transformed Athens into a formidable sea power capable of defending against Persian aggression.
Moreover, the Acropolis itself was once defended by a Mycenaean citadel whose walls withstood attacks for centuries. In response to Persian invasions in the early fifth century BCE, these fortifications were tested and the city saw destruction. However, later developments like the Propylaea not only enhanced the Acropolis's defense but also added to its grandeur.