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. What lies behind most doors are usually hidden by rooves that appear over the area behind the certain door until it is opened. Behind the doors are usually Treasure Chests (which also require certain means to open) and often extra enemies are hidden within the locked chambers as well.
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 they 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, they can open it themselves without any item. Also a few games feture a staff known as the Unlock, this staff allows the wielder to unlock doors from up to two spaces away. Also in the newer games some doors can be opened by sheer force, a unit can simply walk within range of a door and attack it with one of their weapons, the doors have Hit Points and after a certain amount of damage is done to the door it will simply sing open.
Doors are one of the few features of the maps that offer surpises. Since usually you can see most of the enemies before a battle even begins (excluding fog of war maps). Doors however can hide enemy units, treasure chests and many other things within their chambers. Even some bosses hide within locked chambers in certain Fire Emblem games. All considered Doors are one of those often overlooked details in the games that add to it greatly.