Doors are objects that are located in several houses and castles in the Mario series. They would make their first appearance in Super Mario Bros.: The Lost Levels. Doors will usually allow for the player to progress through the level or find hidden areas in the level. If the door is locked, then Mario will have to find a key. They would only be found in Lost Levels after beating Bowser in World 8-4 where he walks into a door and finds Princess Peach.
They would be a common thing in Super Mario Bros. 2. There would be doors that would only appear after a Magic Potion has been thrown and lead Mario, Luigi, Princess Toadstool or Toad to Subspace where they could find a Mushroom or coins(and Yoshi Eggs in Super Mario Advance). There will be doors that are locked in levels and can be unlocked after the player has found a key and carry it back to the door to unlock, while avoiding the Phanto.
Doors would also appear in Super Mario Bros. 3. Super Mario World, Super Mario 64. Super Mario 64 DS. Super Mario Sunshine, New Super Mario Bros.. Super Mario Galaxy, New Super Mario Bros. Wii and Super Mario Galaxy 2. Mario would have to use the doors to explore and visit different areas of Princess Peach's Castle and levels in Super Mario 64 and its DS remake. Doors in platformers can usually be found in the fortresses, Ghost Houses, castles and towers. Large doors will lead to the boss of the world.
Luigi's Mansion would feature several doors, with most of them being locked from the start of the game. Luigi will have to collect keys from chests to be able to open the room. Some doors will also be covered with thorns, this will be because it will lead to a major are though they may trap Luigi in a room as well. When he tries to open a thorny door, he will take damage. There is also Trapdoors in the game and when they are opened, they will slam Luigi into a wall and flattened him. He will take damage and lose coins.
There would be large blue and white doors in Super Mario World 2: Yoshi's Island, and Yoshi's Island DS. They are found throughout Yoshi's Island with some of them being locked and Yoshi will have to find the key so that he can gain access. King Bowser's Castle would have four numbered doors that will spin around and lead to different areas of the level before leading to the final area. Yoshi can also throw an egg at a door and have the others disappear, leaving the door hit to be accessed.