Peach's Castle is the royal castle found in the Mushroom Kingdom and it is the key landmark in the country. Princess Peach and many Toads including Toadsworth live in the castle. It has been a key element in two of the six 3D Mario titles but has appeared in three others. The only 3D Mario game the castle did not appear in was Super Mario Sunshine.
In Super Mario 64 and Super Mario 64 DS Edit
Peach's Castle is the main hub in the game and it is how Mario gets from painting to painting. Bowser takes over the castle and kidnaps Peach, causing Mario to have to explore all the paintings in the castle for Power Stars. At the end of the game, Mario can use a cannon to shoot himself up to the castle and find Yoshi, who is surprised to see him. This feature, however, does not return in the DS remake, as Yoshi is playable.
In Super Mario Galaxy Edit
Once again, Peach invites Mario to her castle with Bowser invading. This time, though, Bowser actually lifts the entire castle into outer space as Mario is helplessly sent flying away by Kamek. Peach's Castle is eventually "rescued", despite almost being destroyed by a massive Black Hole. The castle is then put black in its proper place in the center of the Mushroom Kingdom.
In Super Mario Galaxy 2 Edit
The castle's role continually decreases in the 3D Mario titles, with Galaxy 2 being no exception. It is only featured at both the very beginning and very end of the game. In the intro, Bowser is seen in front of the castle kidnapping Peach once again and Mario walks to the front of the castle so he can get blasted into space. It is also seen in the credits, with a giant cake being seen in front of the castle.
In Super Mario 3D Land and Super Mario 3D World Edit
Peach's Castle merely appears as a background feature in both of the games' opening cutscene though the one briefly seen in 3D World has a slightly different design. Technically, you can enter Peach's Castle in 3D Land if you wait long enough on the title screen. The only function this serves is that it allows players to practice their moves.