New Super Mario Bros.: 3-Fort

We start at the bottom of the fort, where there are three pipes. They are all cannons, but only the middle one will get you anywhere. It will shoot you to the grates above. Go up the grates on the left, wall jump up, and punch the window in the grate to flip it around, and dodge the Amps to reach the first Star Coin. If you do not flip back over, you will still be on that side the next time you climb up a grate. To get back out, go between the top and bottom Amps to the left side and flip back over. Once you get away from the set of four Amps, the second Star Coin is to the left. After you climb a little more, you'll find two doors. The upper left door will take you to the boss door. It's Bowser Jr. Either stomp on him or use fireballs to defeat him. He will take Peach to the castle. Victory gives us access to 3-3. If you go through the upper right door, you'll find a ! Switch that creates red blocks. Quickly climb up them to a door and the third Star Coin. The door will take you to the boss door, as well.