Invincibility
In the entrance level get the toys on the first roof in the following order: Cake, hammer, globe, apple and spout.
Level Select
While playing, type "the little mermaid".
If it works the screen will blink once.
Then press L and use the arrow keys to choose the level.
Secret Levels
In the entrance level, climb the last tower to the top, fall to the right while pressing UP (without getting out of the tower.) Hold the UP key until RoboCod disappears and you will get to the hidden part of the castle.