The limestone hills surrounding Aleppo were, for much of history, considered uninhabitable. There was little water and little fertile land and, in addition to that, the area was prone to multiple invasions and instability. All that changed when, under the stability of the Roman Empire, a few intrepid Romans decided to plant some olive trees. Olive trees are peculiar because it takes them at least 10 years before they bear any fruit, so these Romans planted the trees and then waited impatiently for their fruits. Their foresight paid off when the area soon became a massive olive production area and the Roman farmers became extremely wealthy from the trade money. Their settlements soon turned into small towns with large villas, granaries, wells, and public buildings. However, as the Byzantine Empire started to crumble under pressure from the Persians and Arabs, the region once again became unstable and the settlements were abandoned. The region remained basically uninhabited until the last century when irrigation techniques and other developments made the land habitable.
It is because of the basic lack of human contact that a number of the ruins from the Roman Era are left in surprisingly good shape. Hundreds of villas and other buildings have been located and are free for you to explore. There are so many that the Syrian government cannot possibly keep track of them all and some of them have been incorporated into daily life. You may see a modern Syrian house built on the foundation of a Roman villa, and, in many cases, the locals have just built their lives around the ruins without damaging them. So you will see a Roman house in the middle of a field of eggplants, and you will see local farmers using the same walls as the Romans to mark their fields.
There are far too many ruins for you to see them all. Also, most of them are far beyond the reaches of public transport. If you have your own car, you could spend a whole day driving around and stopping at various ruins. The best way to do visit some of these "dead cities" is by doing it as part of a larger tour to the other sights of the region. Many hotels offer excursions that will stop by one of these villages, most often the one near Qala’at Samaan.
Whichever one you happen to stop at, you will surely not be disappointed. Very little is actually known about many of the structures, so, most of the time, it’s just best to use your imagination. Was this a villa? A church? In many other countries, these ruins would be a star tourist attraction, but in Syria, the land of history, they are just one of many, but they will still fire up your imagination. Some may be a bit appalled by the way people have made their homes among the ruins, but I find that it gives it a particular vibrancy.