In many Mario Games, the story begins in Peach's Castle. In most of them, it is where Peach is kidnapped by Bowser.
Like Super Mario 64, Peach's Castle is the hub world, although this time, Mario must travel to the areas first before he can open up a Warp Zone to the world from Peach's Castle. This makes it easier, since Mario can return in Peach's Castle whenever he wants. A Rabbit boss called Rabbpage appears as the only boss in this area and Mario can defeat him after multiple hits overtime.