The Akkala Citadel Ruins are a location from The Legend of Zelda: Breath of the Wild. They are located in the Akkala Highlands region of Hyrule. It is the ruins of a citadel that was once located in the Akkala Highlands which played an important military role during the Great Calamity. In the middle of the Citadel Ruins, Link can find the Akkala Tower which contains the Sheikah Slate map data for the Akkala region.
After the fall of Hyrule Castle to Calamity Ganon during the Great Calamity event, one hundred years ago, the surviving Knights of Hyrule and Hyrulean Soldiers fell back to the Akkala Citadel. However they where soon attacked by Guardians and monsters loyal to Calamity Ganon. This place was where the Knights and Soldiers valiantly made their last stand against Calamity Ganon's forces, but in the end were slaughtered. It is implied that all the remaining Knights of Hyrule were killed, leaving Link the only surviving member of the order.
A century later, Link can find a Hylian traveler named Nell fighting with a Moblin on the Akkala Bridge Ruins leading to the citadel. Link can assist him by defeating the Moblin and Neil will reward him with Rupees or a random dish or elixir. Neil reveals the history of the Citadel and its role during the Great Calamity. Neil made a pilgrimage to the citadel to honor a Hyrulean Soldier ancestor, but was unable to reach it due to the broken bridge and the patrolling Guardians.