Yemen’s Sira Fortress withstood attacks by the Portuguese and the Turks, but years of war have left the 11th century citadel in disrepair, defaced by graffiti and littered with rubbish. Overlooking the southern port of Aden, Sira sits atop a rocky mountain island in the historic district of Crater, a strategic position that once made it a base for British colonial forces. Around its ancient walls, cigarette butts litter the ground and a visitor has scrawled the words “I love you” on one of the towers of the redoubtable fortress. Yemen’s brutal war has not just killed hundreds of thousands, but…