Palmyra was intended to rival Rome in its magnificence.  It was financed by taxes on caravans which passed between the Mediterranean and Mesopotamia.  Most of the building in Palmyra was done during the third century under the ambitious Queen Zenobia.  She was considered an upstart by the Roman Emperor Aurelian and was taken prisoner by Rome.  What happened to Zenobia afterwards is mostly legend and is open to debate.  

Today, the ruins of Palmyra are considered one of the most extensive and important  historical sites in the world, with much of the city still covered by the desert sands.   The long main street of Palmyra is lined with columns and is known as the Colonnade.  The Colonnade links the Temple of Bel, at one end of the city to the Funerary Temple, at the other end.

Connected to the Colonnade is one of the most remarkable structures in Palmyra - the Monumental Arch,  also known as the Triumphal Arch, Victory Arch, and incorrectly as Hadrian's Arch.  It was built around 200, at the time of Emperor Septimius Severus, to celebrate the emperor's victories over the Parthians. 

The Monumental Arch is particularly impressive from an architectural standpoint because it addresses a problem unique to Palmyra - iIts main streets didn't align with the four points of the compass.  So the arch was designed in order to minimize this effect by adding a duplicate arch at a 30 degree angle in front of the first arch - thus making them appear to be perfectly perpendicular to the street on both side.   The 30 degree angle shows in the model, but the duplicate arch has not survived.  The Monumental Arch had an elaborate decoration and was one of the most lavishly adorned monuments in the city.  Much of the decoration/adornments have disappeared but the overall design is still easy to appreciate.  Monumental Arch is the most popular site of Palmyra.

UPDATE:  In August of 2015 the archaeological remains of Palmyra  came under control of ISIS, a militant Muslim group who purposely targets archaeological sites for destruction.   Within days they destroyed Palmyra's Temple of Bel and Temple of Baslshamin.   The Gene Gill miniature model of the Monumental Arch was finished October 3, 2015.    On October 4, 2015, ISIS blew up the famous Monumental Arch.