“A key used to open a door.” —In-game description, Shadow Dragon and Shin Monshō
The Door Key (扉の鍵 Tobira no kagi) is a key that recurrently appears across the Fire Emblem Series. The Door Key is, in essence, a skeleton key that is specifically designed to allow its holder to bypass all locks imposed on Doors. However, due to there being a lack of actual doors in most chapters, alongside the possibility to eliminate doors by breaking them or picking their locks in several games, the Door Key is thus often regarded to be an obsolete item. This is especially true towards the concluding stages of any given title, junctures during which the player should have recruited at least one Thief character, who is able to unlock both doors and Chests without the aid of keys.
Door Keys are usually made available in the early chapters of any given title that feature doors impeding progress. This is due to the fact that Thief units are often not available during such junctures, necessitating Door Keys to function as substitutes for them. Enemy units positioned near doors in such chapters usually possess enough Door Keys to unlock all of them.
Door Keys are also commonly dropped alongside Chest Keys, items that are specifically used to unlock chests. In such situations, the doors in question usually function as barriers blocking access to rooms containing chests.