At its heart lies the emblematic Al-Nuri mosque, where jihadist supremo Abu Bakr al-Baghdadi declared a caliphate in June 2014 after his forces seized Iraq's second city along with swathes of territory extending into neighbouring Syria.
Mosul's Old City threatened as showdown looms
Mosul's Old City, where Iraqi forces are closing in on the Islamic State group's final urban refuge in Iraq, is an ancient maze of narrow alleys.
Smoke billows behind the emblemati Al-Nuri mosque in Mosul's Old City, where Iraqi forces have launched an assault on the Islamic State group
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Baghdadi's cross-border "caliphate" has been shrinking steadily since mid-2015.
The loss of Mosul would leave Raqa, in Syria, as the group's only major urban stronghold.
Perched on the bank of the river Tigris and protected for centuries by 11th century ramparts, medieval Mosul was a key meeting point for merchants from India, Persia and the Mediterranean.
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Today, the three-square-kilometre (one-square-mile) district is a maze of alleys lined with stone houses, small shops and the workshops of local carpenters, weavers and metalworkers.
It contains numerous markets, churches and mosques, the most emblematic of which is the Great Mosque of Al-Nuri.
Baghdadi's only known public appearance there heralded the most ambitious and brutal experiment in modern jihad, a period marked by mass murder, slavery and attempts to commit genocide.
The mosque takes its name from Nureddin al-Zinki, who ordered it built in 1172 after unifying Syria and parts of northern Iraq.
A predecessor to Saladin, Zinki was a Muslim hero in what he labelled a jihad (holy war) against the crusaders.
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Present-day jihadists often borrow his rhetoric, referring to western forces as "crusaders". One influential rebel group in Syria has even named itself after him.
Emblem in peril
His mosque in Mosul was largely dismantled and rebuilt last century as part of a renovation project, but its iconic leaning minaret, which locals dub the "hunchback", survived.
Decorated with geometric brick patterns, it is an emblem not just of Mosul but of Iraq and appears on the 10,000 dinar bill.
After seizing Mosul, IS fighters hung their black flag from the top of the 45-metre (150-foot) tower.
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While the jihadists destroyed priceless historical sites in other parts of their "caliphate", witnesses say the "hunchback" was saved after locals formed a human chain around it to protect it.
Historians and architects fear that fierce fighting in the Old City will imperil its heritage as well as the mosque's fragile minaret.
In 2012, UNESCO warned that the minaret was suffering "serious structural weakness".
"It is feared that the leaning minaret, that has brought fame to the city of Mosul, may soon collapse if measures to save it are not taken," it said.
A UNESCO-led effort launched in June 2014 to stabilise the structure was interrupted just days later as IS seized control of Mosul in a lightning assault.
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In a "desperate plea" published in March, Iraqi architect Ihsan Fethi called for the Old City to be spared.
It contains "monuments and houses of historical and architectural value that are among the most remarkable in Iraq and the region," he said.
He urged Iraqi and international coalition forces not to resort to "any sort of indiscriminate artillery, bombing... or any similar heavy weaponry" in the battle for the Old City.
"We have already seen that in (Syria's) Aleppo and elsewhere," Fethi said.
"If the city is liberated at the price of destroying Old Mosul, it will be a hollow 'victory.'"
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