The nuraghi are truncated cone-shaped, squat tower-shaped structures built by the Bronze Age and Iron Age peoples of Sardinia between 1500 and 500 BC, consisting of rows of stones placed on top of each other without a binder, and made with so-called cyclopean masonry. Approximately 7,000 nuraghi have survived on the island; these usually belonged to and protected a smaller territorial unit. The more complex nuraghi are real castles, where the central part is protected by corner towers and thick ramparts; the rooms located on several levels can be reached by internal stairs. In the central part of the Mediterranean region, the nuraghi civilization represented the megalithic tradition.