Well truthfully, it depends, their is a couple of islands that have castles on them, but if you want to know about the castle in Vampires Curse, you have to solve a few things first before you get to it.
in Mordred's castle
under the library in the castle
In front of Hades castle.
keep going right if it doesn't work you'll see arrows to the castle
You get the chese for a mouse in the left room in the destroyed castle in astro knights in poptropica
well you find it in the castle near the mcm and press the brick and a passage will open
Nothing
get the moldy cheese and get the mouse in the castle
To get to the queens castle in poptropica you go to early poptropica then you go to poptropica towers.after that keep walking left until you see a purple building and it says rooftop resturuant you go all the way to the top and you will see a vine.Climb the vine an then just keep going right
The arrow is at the castle
if you mean in astro knights, the mcm is in the library in the castle
You get the cordniteres on Astroknights island from the castle from the King and Queen.