This ruin is in Addy's subconscious and does not exist in the real world.
The Ruins only exist because Addy had suppressed that she was forced to kill her mom on the first night of the outbreak.
She dreamed that she and Mack Thompson arrived there after leaving an idyllic park.
While at the ruins, Mack experienced the same zombie attack over and over, unable to kill it. Finally, when he involved her in the zombie kill, she broke through her block and remembered killing her mom and her brother Michael, since they were both zombified.