The Hubworld

Probably the defining aspect of this game. The hubworld is Peach's castle, and one acesses each level through paintings in every room (and some other hidden places). Some rooms are locked behind the number of stars one currently has, however, the game is as far from linear as it gets. One begins the game outside the castle which allows for one to get comfortable with the movement mechanics before tackling the castle. After that one doesn't need to follow a strict path to advance, one can for example finish most of the stars in the first room or explore the castle and tackle the stars at one's leisure.