The door to Mordred's secret underground laboratory is under the two hay bales near the mill.
(see related question)
go to the secret door
the door that's locked
You jump in between the middle and then you push them and you'll find the secret door to mordred's layer then you use the key
In the palace the right door
you have to finish the puzzle then click on the door
you want to arrow the door on top of the castle
On the first floor of the castle, left side door. There is a secret message there, as well as a mechanical mouse. (see related question)
the guy at the door fives it 2 you
The Cosmic Symbols are the password for the secret society. Use them to open the secret opening on the village fountain. Go to the fountain in the village. On the plaque, press the three symbols and the Sun, and it will open a secret door.
A "trap door" is one that opens unexpectedly underneath you. The only such door is on 24 Carrot Island. There are many other door in floors on Poptorpica, such as the laboratory or secret stairway on Astro Knights, the Dragon Face door on Counterfeit, or the secret room on Big Nate Island. None of these are trap doors. (see related questions)
On the secret planetoid (X11 Y82), you can draw the Golden Key from the crystal, opening the path to the castle in a far galaxy. To enter, you will have to solve the door puzzle by the Binary Bard. (see related question)
half moon.... planet with rings....star....sun