Like so many who traveled to the Orient in the 18th century, French artist Louis-François Cassas came from an upper-class family. His father was a marquis and royal land surveyor. In 1785, Louis-Francois spent two months in Palmyra, drawing virtually all of the ruins of the legendary cultural center of the Ancient World.

Cassas encountered nothing but ruins in Palmyra; however, instead of drawing what he actually saw, he tried to reconstruct the ancient city in its former splendor on paper. The Wallraf-Richartz Museum in Cologne has 123 of his sketches in its collection. Some of them were restored for the exhibition "Palmyra: What's Left?," which also reflects on the current terrorist destruction of these sites.

Palmyra is located halfway between Damascus and the Iraqi border. Erected between the first and the third centuries AD, the monumental buildings were well-preserved, silent witnesses to the wealth of the Greco-Roman city. Trade caravans brought spices, precious gems and cloth to the independent city.

Palmyra went with the times, mixing architecture in the Greco-Roman style with indigenous elements and ornamental flourishes. The semicircular Roman Theater has a stage facade designed like an oriental palace. Plays in Aramaic were shown there.

Artists and architects have aimed to revive the spirit of the antique city as early as the 15th century. Cassas' perspective was unique: Using different colors, he distinguished the actual architecture from the imagined building in his drawings. Black depicted reality, while red marked his reconstructions.

Cassas' drawings offer insights into Palmyra's burial rites. The dead were entombed in burial towers, three to four stories high, with up to 42 sarcophagi per level, each elaborately decorated with the likeness of the deceased.

The elaborate ornaments in the towering necropolises demonstrate that the burial towers were reserved for wealthy residents of Palmyra. It is not immediately clear what actually existed at the time Cassas made his drawings - and what he added.

The temple of Bel was built toward the end of the second century AD under Roman rule. Bel was the local equivalent of the Greek god Zeus. The architecture combined Greek and Roman building traditions, with additional oriental flourishes.

The approximately 2,000-year-old temple of Baalshamin, one of the most complete ancient structures found in Palmyra, was also demolished by "Islamic State" militants. Baalshamin, the "Lord of Heaven," was one of Palmyra's supreme deities.

The Great Colonnade was the city's main avenue. There were shops to the left and right, as well as the Agora market place, a theater and the roman Diocletian bath complex. As the IS pursues its destruction of ancient sites, what will finally remain of this cultural heritage remains to be seen.