The "dead cities" generally refer to the Byzantine-era ruins scattered around the various mountain regions between Hama and Aleppo, the bulk of them being in Jebel Zawiyeh (north of Hama), Jebel Samaan (including St. Simeon Monastery), and Jebel Barisha. Serjilla, al-Bara, Jeradeh and Ruweiha are all part of Jebel Zawiyeh,. Ebla is not considered one of the dead cities, and is far more ancient (Bronze age, I believe). Qasr ibn Wardan, while of the Byzantine period, is generally not classified as one of the dead cities either.
I've been to over 50 of the dead cities. Serjilla and al-Bara are certainly among the most impressive, and can be visited in a single day on your own. There are 3 minor dead cities between Serjilla and al-Bara: Bauda, Majlaya, and Batrasa, that can be explored as well. One of the most impressive sites in this region, larger than Serjilla and in a great state of preservation, is Shinshirah. It is south of Serjilla, beyond another smaller site called Rabiaa. Very few foreigners know about Shinshirah, but it seems a popular picnic spot with local villagers. If you ask the caretaker at the ruins of Serjilla, he can direct you towards all of these sites. I'd advise against visiting Ruweiha: it is out of the way, the locals inhabiting the site aren't particularly welcoming of foreigners, and the site is not that well-preserved.
There are a greater number of dead cities west of Aleppo, in Jebel Barisha and Jebel Samaan, and they are in denser concentration. In the valley below Qalaat Samaan (St. Simeon Monastery) you can find about a dozen sites: Deir Samaan, Taqleh, Qatura, Zarzita, Sit al-Rum, Rafadeh, and plenty to occupy another day of exploration. Higher up the mountain, east of Qalaat Samaan in Kurdish country, are numerous other sites: Burjkeh, Fafertin, Burj Haider, Kharab Shams, Kaluta, Brad, and others (names escape me at the moment). Conveniently, hiking paths have recently been established in this area with maps posted to guide visitors. It is a great region to explore, and the people are wonderful.
Further west of Aleppo, and slightly south, is Jebel Barisha. There is not any particularly well-known site in Jebel Barisha, and they are all fairly minor, but it is a beautiful area to explore. One nice hiking route is from the main road (leading to Harem) to Baqirha, Deir Qeita, and Babisqa. Though they are small sites compared to al-Bara or Serjilla, they are still impressive and have commanding views towards Turkey. Baqirha also has a Roman-era temple, in addition to the Byzantine ruins. Other sites in Jebel Barisha include Maaz, a small church in Kafr Darian, and literally dozens of others that I can't remember off the top of my head. Monuments of Syria has many of these listed, while others can be found in Syria: A Historical and Architectural Guide.
A bit further to the west of Jebel Barisha is a small mountain range called Jebel al-Aala, that has another dozen of so interesting sites. The most famous site in that area is Qalb Lozeh. Others include Qarqbizeh, Behyo, Beshlandiyeh... I put together my own (half-finished) guide to the dead cities in Jebel al-Aala: http://syriaphotoguide.com/jebelalaala/
Guides to the other regions were in the works, until I got distracted by real life and the need for a paying job. ;) Hopefully I'll get them finished some day.
Hope this helps. You can contact me if you have any questions, regarding transportation or what have you: dan at syriaphotoguide dot com.
