Doors (ドア) are a feature of certain battle maps in the Fire Emblem series.
Doors, as the name suggests, are simply doors. In the game's maps they appear as locked and usually require some means to open them. Roofs usually hide what lies behind most doors until the door is opened; unlocking the chambers may reveal Treasure Chests, and, often, lurking enemies. Doors can be opened several different ways. In the older games a character either needed to be carrying a Door Key (an item made specifically for opening doors) or had to have a Thief unit with a Lockpick item open the door.
In later games several other means of opening doors have been created. In the newer games Thieves (and promoted Thieves) no longer need the assistance of the Lockpick item to open a locked door, but can open it themselves without any item. A few games feature a staff known as the Unlock which allows the wielder to unlock doors from up to two spaces away, and in some games some doors can be opened by sheer force, swinging open after the heroes wipe out the door's Hit Points by attacking them.
While on most maps, excluding Fog of War maps, you can see most enemies before battle even begins, doors can conceal enemy units, treasure chests, or, in some games, bosses, or other secrets, leading to surprises which greatly enhance gameplay.