Over four millennia, Babylonian, Egyptian, Persian, Greek and Roman influences crossed paths in Syria. Today, precious cultural sites from these eras are being damaged as the Syrian civil war takes its toll on the country's cultural heritage. The United Nation's cultural organization UNESCO is concerned for the country's cultural heritage.

In 2013, UNESCO listed six unique World Heritage sites in Syria as being in danger, including the historic center of Aleppo. Aleppo has a long history: as a cultural hub on the Mediterranean Sea, it is mentioned in documents dating back to the 19th century B.C.

UNESCO also recognized Aleppo's historic center as a World Heritage site because of the ancient souk, the largest covered market in the Middle East. The bazaar covers an area of about 350 hectares with several streets and hundreds of shops. But even the five-kilometer long wall surrounding the historic center was not able to protect the market: it burned down during fighting in 2012.

Over the course of the civil war, cultural sites have been misused as strategic bases - among them the Citadel of Aleppo, a fortress on a hill towering above the old city. The Seleucid Empire, a dynasty that followed Alexander the Great, erected the fortress in the 4th century B.C. Greeks, Romans, Persians, Byzantines and Ottomans have also built temples on the hill.

The old town of Damascus, inhabited for the last 4,000 years, was also named on the UNESCO list. Before the start of the uprising against the Assad regime, the old town with its markets, restaurants, churches and mosques was one of Syria's main attractions. Bombs exploded here in June 2013, the first time the area was part of a larger attack.

The oasis city of Palmyra is also at risk. The Monumental Arch is still standing, but some archaeological excavations sites have been looted. Palmyra is one of Syria's architectural landmarks. The boulevard with its Corinthian columns, the arch from the time of Septimius Severus and the walls of the Temple of Baal have now been marked by bullet holes.

The Krak des Chevaliers, a well-preserved Crusader castle in the Homs Gap, is also in the middle of the war zone. Crusaders on their way to Jerusalem first reached the castle in 1099. The castle's current state is contested - the Free Syrian Army is supposed to have used it as a base, but Syrian government troops apparently took the castle in March 2014.

The amphitheater in Bosra was once considered to be one of the best preserved Roman theaters in the world. In the 12th century, it was converted into an Arab fortress. In modern times, musicians and orchestras from around the world valued its special acoustics. But fighting has has apparently turned the amphitheater into true ruins.

Syria's so-called "dead cities" continue to be in danger. These village settlements in northern Syria have numerous houses from the Byzantine era, and the remains of some ancient buildings in Jerada were very well preserved until the start of the civil war. Since the fighting began, they have been partially under fire, and have also been looted.

Syria's museums are also at risk. Many are in the middle of combat zones - such as the museum in Idlib, which houses a majority of the priceless Ebla clay tablets. Since 2011, a number of valuable pieces have been moved from museums in Damascus and Aleppo and stored in a safe in the Syrian Central Bank. But most museums are defenseless against the looting.

This part of the World Heritage site in the ancient city of Palmyra in Syria no longer exists: "Islamic State" militants have blown up the 2,000-year-old temple of Baal Shamin. The inner area of the temple and the columns collapsed through the explosion. The rest of the ancient city remains strongly at risk.

The ruins of Palmyra lie right in the middle of the Syrian desert. The once prosperous metropolis was surrounded by palms - hence its name - and for centuries was a stop for caravans traveling to the Silk Road. The settlement was a center of wealth and trade. But, gradually, the golden age faded, and sand blew over the city. The ruins were later excavated, and given World Heritage status in 1980.

In the 1st century AD, the Palmyrenes built a grand Roman-style temple for the deity Baal. It formed the center of religious life in Palmyra, which joined the Roman Empire under Emperor Tiberius some time after 14 AD. It is scarred with bullet holes - stark reminders of the ongoing Syrian civil war.

Created in the 2nd century, the Great Colonnade stretches on for more than a kilometer (0.6 miles). Spices, perfumes, precious stones and other treasures once passed down this magnificent colonnaded boulevard. The avenue's entrance is marked by Hadrian's Arch, built in honor of the Roman Emperor Hadrian. It's a fine example of the Greco-Roman style, extremely popular at the time.

The Tetrapylon of Palmyra was built on a crossroads. The four groups of slender pillars, each supporting an alcove, were made of red granite brought in from the quarries of Aswan. Each one used to house statues. Today, almost all the columns are replicas. Only one is an original.

Palmyra bore many characteristics of a Greco-Roman city. It had a portico, thermal baths and an amphitheater. Many oriental dramas were performed on this stage. Unfortunately, the plays, written in Aramaic, haven't survived. In addition to being a theater, the arena was also used for battles between gladiators and animals.

Some 200 statues of important individuals once stood here, taking up honored positions in the porticoes of the agora, or main square. In the agora's southwestern corner the remains of a building where the city council likely held its meetings can be seen. The council was made up of representatives from influential merchant families, responsible for shaping the fortunes of the desert city.

There are a number of burial grounds just outside the city gates. Large families built tall towers housing ornate sarcophagi and tombs big enough for several generations. There are also many underground gravesites decorated with rich architectural flourishes and frescoes that hint at the daily life and wealth of that period.

In 300 AD, Palmyra became a military base, and came under the power of a string of different rulers. The golden age faded, and the city's splendor was covered up by the desert sand. The city's ruins survived the civil war raging in the country since 2011, but now, the ancient city is under threat from "Islamic State" militants, and UNESCO fears it could face a tragic fate.