The East Post Ruins are a location from The Legend of Zelda: Breath of the Wild. It is located next to the Outpost Ruins. Being that it was a Hyrulean military outpost, the complex was most likely targeting by Guardians that sought to destroy any and all forces that opposed Calamity Ganon before they could mobilize.
It is unknown what the outpost’s true purpose was before the Great Calamity other than to serve as a regular military outpost and possibly a checkpoint. However, during the Great Calamity, the outpost was overrun by Guardian forces, in which the soldiers and guards stationed there put up a valiant defense that ultimately failed, leading to the outpost’s destruction. The remaining survivors most likely fell back to Akkala Citadel, where the remaining members of the Hyrulean Guard combined their forces in a last stand against the Guardians.