Achilles: Legends Untold

Cellars are marked on the map by a small door symbol (the way they look in the game world is exactly as they appear on the map), while ruins are large archways. In both instances, the icons will be in color once you have discovered them and then go grey once you have visited them by entering for the first time. Oddly, the exact opposite of shrines...

You do not need to "complete" the cellars or ruins for this achievement, just enter and leave - but be sure to complete them as you will often find rune chests or notes inside these locations, which will help you work toward other achievements.