On Stage 5, in the first room with all the ninjas, there are three secret routes you can take other than the normal route. You can access them by killing all enemies here. These rooms have more items than the normal rooms. The way to get to the rooms are as follows:

1.) Above the door where you follow your normal route, there is a white wall in front of you with cracks near the top. Punch it to access Secret Route 1.

2.) At the bottom of the screen is a very unlikely secret that most of you won't suspect until reading this. Go to the bottom center part of this room and punch a few times. You'll eventually bust a hole in the floor leading you to the basement of Stage 5 and also Secret Route 2.

3.) Go to the top of the screen and go up to the red walls. There is a certain red wall with a few cracks in it. Punch it to access Secret Route 3. Beware. There are a lot of enemies here, so be careful.