Complex sentence: The Darial Pass was historically important as one of only two crossing of the Caucasus mountain range, the other being the Derbent Pass, and has been long fortified--at least since 150 BC. Ruins of an ancient fortress are still visible.
Split into simpler sentences: The Darial Pass was historically important as one of only two crossings of the Caucasus mountain range, the other being the Derbent Pass. As a result, Darial Gorge has been fortified since at least 150 BC. Ruins of an ancient fortress are still visible.