This stage is Fort Fire Bros (Castle-1). This complete walkthrough will provide you with a 100% video playthrough and a screenshot of where you can find all the green stars. We will also note a secret 1 UP.
Game Play Walkthrough Video
Green Star 1
You won’t miss this green star. The green star sits in the middle of the H shaped clear pipes. Consider taking out the Fire Bro before getting this star to avoid getting damaged.
Green Star 2
After hitting the check point, climb up to the platform behind the flag using invisible blocks. Enter the pipe and start a mini game. Your goal is to get the ball to the switch to release the green star. You will have to time your jumps perfectly to manipulate the ball. Check the video walkthrough if you are having trouble with the green star.
Green Star 3
When you get to the Hallowed green star, take out the Block-steppers first by using the POW blocks. Once they are out of the way, trigger the hallow green star and collect all the coins to claim your green star.
When you get to the rotating fire bar with a ? block as the center. Ground pound the upper one to raise a block tower. The stamp lies at the top of the tower. Note that you can use the tower to snipe some of the Goombas which make it easier to kill.
Other Notable Secrets
Right after getting the first green star, look for the hidden 1UP block to the left.