I have visited Roman-era ruins in about a half-dozen countries. Palmyra/Tadmur was by far the best one I have ever been to. Unlike every other Roman site I have seen, the ruins of that ancient city stretch out for an area that is city-sized, giving a real sense of the entire ancient city. Plus, the site has pre-Roman tombs, and an abayyid-era citadel. It's hard to imagine how much will be lost if ISIS conquers the site.
