Throughout the years the Castle has varied in appearance greatly, it is still quite likely caused by variety in artistic direction, and not actual redesigns of the Castle. The following list describes the appearance of Princess Peach's Castle, game by game, in the order of which each game was released.
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In Super Mario 64, the Castle and its surrounding areas make up the Hub, providing access to the game's Worlds. It is in this game that we are given our first, in-depth look at the Castle, establishing many of its recurring themes; Including it's stain-glass Portrait of the Princess, it's bridge & moat, red-tile roof, and it's light grey brick exterior.
In Super Mario Galaxy, Bowser uses the power of the Grand Stars to transport the Castle, with Princess Peach within, to the Center of the Universe. In appearance, the Castle looks like a modernized version it's Super Mario 64 design, maintaining all major elements from its Predecessor.
In Paper Mario, the Castle is nearly identical to its design in Super Mario 64, differences include the stain-glass window being replaced by a bronze Super Mushroom Emblem, the Castle's flags changing in color from red to blue, and some of the spare rooms being put to use; For example, there is now a door at the top of the stairs in the Main Hall that leads into the Ball Room, which in turn connects Peach's Room, and the Castle Balcony.
It only appears briefly at the beginning of the game, when Mario and Peach are taking a walk, Lakithunder attacks the Castle, distracting Mario long enough for Bowser Jr. to kidnap the Princess. It looks aesthetically identical to its appearance in Super Mario 64.
The castle appears in Mario & Luigi: Superstar Saga as the first area explored, the first battle of the game with Bowser takes place here. After "Peach" releases a chunk of speech bombs, the castle explodes (but isn't destroyed). The outside of the Castle is known as Toad Town Square. The castle doesn't play much of a role after this. It can be seen from Hoohoo Mountain in the Beanbean Kingdom and is also seen in the ending credits. The Castle's design in this game is orange and white.
The castle again serves as a hub area in which players can buy, sell and trade as well as access the separate levels of the game via portals. The castle is also where the Mario bros. learn vital techniques such as Spin jump and Bros. Ball. It is taken over by the Shroobs during the events of Mario & Luigi: Partners in Time.
It makes a major appearance in Mario & Luigi: Bowser's Inside Story, when the Castle was temporarily taken over by Fawful. The castle was given the ability to turn into a giant robot by Fawful. It fought with Giant Bowser. It had the ability to turn invisible and shoot black holes. In this game, the Castle is fully magenta.
Princess Peach's Castle appears again in Mario & Luigi: Paper Jamas the first area of the game. The appearance has changed again, looking more like Super Mario 64's version but smaller and a bit more brighter and pinker. The attic is also seen, where the book containing the Paper Mario universe is held.
In the remake for Mario & Luigi: Superstar Saga, the Castle plays a fairly similar role to the original with a few changes. The most notable change is the exterior. It's colors have been changed to match that of Dream Team and Paper Jam's but the layout still remain the same as in Superstar Saga. Also, Toad Town Square has now been renamed to Peach's Castle. Another thing is that Peach's Castle is no longer visible from Hoohoo Mountain due to being blocked by the Clouds.
In the beginning cutscene of Mario Party 4, Princess Peach's Castle is shown. It resembles the same Princess Peach's Castle from Super Mario 64, with minor changes, like the windows are different from the original. It even shows her castle during the main menu in Mario Party 9.
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The Castle is also used as a Stage in Super Smash Bros. Melee, and has a bit more of a brownish brick color to it rather than the more white color that has been seen in other Mario games. Players fight on the roof of the castle in this game. The castle also appears as an unlockable trophy throughout the game.