The Acropolis of Athens is an ancient fortress perched on a rocky outcrop above the city of Athens. It has the remnants of numerous ancient monuments of great architectural and historical value, the most renowned of which is the Parthenon. The name "acropolis" is a generic phrase, and there are several acropoleis throughout Greece. The word comes from the Greek ἀκροπόλις (arkópolis), meaning "rocky place". In modern usage, it refers to the fortified hilltop where an ancient Greek or Roman city would be built. The Athenians called their city "to kitein akrogoreio", which means "to build a citadel on an akros (highest) point". The location was chosen because it was visible from far away and could guard against invasion from the sea. The first settlers here apparently used timber for building purposes, but the city later had stone buildings constructed by skilled masons. Many of these stones are still present today and can be seen scattered all over the Acropolis. In addition to being used for military purposes, the Acropolis also served as a place of worship for the Ancient Greeks. They believed that the gods lived in heaven at a place called Olympus, and so this area of Athens was considered holy ground. On the top of the Acropolis are four temples, two to Athena and two to Ares. These were places where sacrifices would be made on behalf or good luck, victory, and other offerings.