Complex sentence: The mountains also acted as a natural defense against nomadic invasions from the North, as only four passes cross the mountains, although in the late 1990s a railway tunnel was completed, thereby easing travel across the range.
Split into simpler sentences: The mountains also acted as a natural defense against nomadic invasions from the North, as only four passes cross the mountains. In the late 1990s a railway tunnel was completed, thereby easing travel across the range.