The Hyrule Garrison Ruins are a location from The Legend of Zelda: Breath of the Wild. It is located in the area southeast of the Central Tower and north of Windvane Meadow in the Hyrule Field region.
EX Side Quest
As part of "The Master Trials" DLC, during the side quest "EX Treasure: Phantasma" and is where the bandit Misko hid the Phantom Greaves after he stole it from Hyrule Castle. After reading Misko's EX Journal, Link can find a EX Treasure Chest buried near a broken Guardian in the eastern part of the ruins which contains the Phantom Greaves.