The Western Romanian Carpathians are the lowest of the three ranges and are fragmented by many deep structural depressions. They have historically functioned as gates, which allow easy passage but can be readily defended. The most famous of these is the Iron Gate on the Danube. The Western Romanian Carpathians are the most densely settled, and it is in the northernmost area of this range, the Apuseni Mountains, that permanent settlements can be found at the highest altitudes.