“A key used to open a door.” —In-game description, Shadow Dragon and Shin Monshō
The Door Key is a key that appears across the Fire Emblem Series. The Door Key is a skeleton key that allows its holder to bypass all locks on Doors. However, since there are no doors in most chapters and doors can often also be opened by breaking them or picking their locks, the Door Key is often not very useful. This is especially true near the end of any given game, since 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 game that features doors impeding progress. This is due to the fact that Thief units are often not available at that time, so Door Keys 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.