Beirut — 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....