A general view shows Aleppo's historical citadel, where today Bashar al-Assad’s forces have taken position inside to shell their enemies, and Syrian opposition fighters say they are desperate to capture it. (JOSEPH EID/AFP/GETTY IMAGES)

A Shiite king ruled northern Syria more than a millennium ago from behind the towering walls of the citadel in the city of Aleppo. In later centuries, Arab armies repelled medieval crusaders from the hilltop fortress, Mongol invaders damaged it and Ottomans used it as a military barracks.

By 2011, the citadel had settled into what seemed a comfortable retirement as a UNESCO world heritage site and tourist attraction, illuminated at night by artistic ground lights and surrounded below by the bustling cafes of Aleppo’s old city.

But today, in the third year of a bloody civil war that has killed more than 70,000 Syrians, the hulking citadel has resumed its strategic role of earlier eras. President Bashar al-Assad’s forces have taken position in it to shell their enemies, and Syrian opposition fighters say they are desperate to capture it. For both sides, what was true in war then is true now: Those who control the citadel have the power to alter the front lines.

Modern Syria is dotted with medieval castles and citadels, many built high upon the ruins of earlier Roman or Mesopotamian dynasties in an archaeological landscape that experts say is among the richest in the world. But as the fortified structures gain new strategic purpose in Syria’s devastatingly modern civil war, archaeologists worry that what withstood ancient armies and earthquakes may now fall victim to airstrikes, shelling and other forms of 21st-century warfare.

Because of limited access, archaeologists and other experts say it is close to impossible to confirm reports of damage and looting to Syria’s castles and citadels, including the famed crusader castle Crak des Chevaliers, whose south wall has been nearly destroyed in the fighting, according to Syrian rebels.

But it is certain that they and many other historical and archaeological sites “have been affected by violent fights or occupation by armed forces for military purposes,” said Veronique Dauge, chief of the Arab States Unit at the UNESCO World Heritage Centre.

Both the rebels and the Syrian government have pledged publicly to protect the nation’s ancient structures. But they are intensely battling for their control.

In Aleppo, the citadel has proved critical over months of fierce fighting. Syrian opposition fighters say regime snipers have staked out positions in the arrow slits of the ancient fortress, rendering the hilltop impregnable and allowing the snipers to cement a front line that roughly dissects the city in half — one swathe controlled by rebels, the other by the state.

Near the city of Homs, in the strategic Orontes River valley, which has served as a battleground of clashing empires for more than 4,000 years, Syrian rebels say they only recently routed regime troops from the heavily fortified walls of Crak des Chevaliers — one of the Middle East’s most fantastic crusader castles.

In the center of the old city of Homs, the citadel has changed hands at least three times in recent months, although some fighters say they only managed to hold it for a day. There, opposition forces say the regime has used the fortress to maintain its stranglehold on one of Syria’s most important and most virulently anti-Assad cities.

“If the rebels got control of this citadel, it would mean that the direct shelling on the areas in old Homs would stop,” said Jalal Abu Soliman, a member of the Local Coordination Committees, a Syrian activist group. That, he said, would allow the rebels to take full control of the city.

Opposition activists in Hama say regime forces occupying the medieval al-Madiq citadel have maintained an upper hand by using the structure to shell villages to the north that are sympathetic to the opposition and might otherwise rise up to fight.

Wars often carry eerie parallels to a region’s earlier history, but Middle Eastern historians are fearful about what the current fight may bring to Syria’s rich historical sites.

In Aleppo, Syria’s former commercial capital and a vibrant gem of the region’s historical heritage, successive battles between forces loyal to Assad and rebels have left the Islamic old city ravaged by fire and torn down by artillery shells. Assad’s tanks blasted through the 11th-century minaret of a treasured mosque, activists say. And Aleppo’s 14th-century souk — once a covered maze of well-preserved stone and wooden shops — has gone up in flames.

“We create these heritage sites in times of peace, and then we destroy them much faster than we built them,” said Helga Seedan, an archaeologist at the American University in Beirut.

But even amid the destruction, Aleppo’s citadel is still standing, and it remains a prize worth fighting over.

“It’s a center for more than a hundred snipers,” said Majed Abdul-Nour, the alias of a local opposition activist interviewed via Skype.

“There have been many attempts by the rebels to liberate the citadel,” he said. But the high ground and high walls, and the bullets fired so easily through its arrow slits, he said, “make the mission to liberate it too difficult for now.”

Online images that have circulated in the past year show shell damage and alleged looters digging holes around the al-Madiq citadel in Hama province, the Aleppo citadel, Crak des Chevaliers and the Palmyra citadel in Syria’s central desert — the latter three of which are UNESCO world heritage sites.

But while Syrian officials have acknowledged some of the destruction, they have cast the blame for it on “terrorist groups,” the catch-all phrase that the state has assigned to its opponents, whether peaceful protesters or armed Islamist extremists. Opposition groups attribute the damage to the regime .

In Hama, activists said government forces had transformed the third-century al-Madiq citadel, which towers over acres of green farmland, into a military garrison that the troops have to batter Sunni villages nearby.

“Inside the citadel there are heavy and mid-range weapons,” said Ahmed Radoun, a local activist. But the rebels have never tried to storm it because displaced local residents are believed to have also sought refuge behind its powerful walls, he said.

Even the regions that are contested today echo the battles of a bygone era. The Orontes River valley, which forms part of the border with Lebanon, has been a strategic passage for so long that Egypt’s pharaohs fought for it, later inscribing the scenes of battle across the walls of their temples.

“All the kingdoms of settled Syria wanted to control the famous route that is known as the Homs gap between the coast and the Orontes valley,” said Helen Sader, an archaeologist at the American University in Beirut. “That is why the crusaders built the Crak des Chevaliers on this route to protect it.”

And that is why Syrian regime forces and opposition fighters say they are fighting hard for it today. Several months ago, opposition fighters say they overran Crak des Chevaliers’ sophisticated walls and turrets, which they say the regime had used to shell the neighborhoods below.

“The regime knows that importance of this castle,” said Abu Hamza al-Homsi, a local activist. “These stones are like shields for any fighter inside.”

But the fortress has continued to weather government strikes, activists said. Another fighter, who gave his name as Qais al-Hussni, said that almost the entire southern wall of the fortress has been destroyed.

As the weaponry and rules of modern warfare evolve, so, too, may the towering symbols of Syria’s heritage, until they fade into the so-called tells — cone-shaped mounds of disintegrated ruins — on which they were built, for a future generation of experts, looters and warmongers to comb through.

“It’s a crossroad of cultures. It’s a crossroad of armies. It’s a crossroad of trade,” Sader said.

“This is the importance of Syria. And this is what made it so attractive to so many powers — and it still is,” she said. “We just hope that this will end very soon, before they destroy everything.”

Suzan Haidamous contributed to this report.