Once you have obtained 10 Stars go to the lobby of the castle, aka the first large room you're in when you first enter through the front door.

Stand in the center of the lobby, on the picture of the sun on the ground, and go into first person mode. Look straight up towards the shining light and you will be teleported into the red cap world. Beat the world to unlock the cap in all the levels that contain a red cap box.

get 10 stars like he said

Get 10 stars

Get 10 Stars

10 stars

10 stars broseph

To get the Red, Wing Cap - first you need to get 10 Stars, then a ray of light will appear at the entrance inside the Castle. Walk under the light and then look up at it to be transported to the Bonus Stage.

I have no idea because this is my son's game. But he really enjoys it and I'm sure you could google it to find the answer. :)

I concur that all you have to do is get 10 stars.

Yup, after 10 stars you can enter a secret level in the main castle area. You need to collect all 8 red coins in this area to switch on the cap in the game.