To get the key to the top door. You must have at least 30 stars. When you do, enter the star door in the basement and jump in the weird blue wall and get another star, when you come out there will be a hole in the floor which you must jump in and beat the bowser in. Its a lava world by the way. When you beat him take the key he gave you and run up to the second floors door and open it! :)