You will get it from the secret society that meets under the fountain.
Take the message from the Maid's room (castle first floor left) to the loft inside the mill, where a goth girl will give you the Cosmic Symbols password card. Go to the fountain plaque and press the three symbols, and the sun, to enter the chamber. One of the robed members will give you the key.
The small key in Astro Knights Island is for Mordred's secret hideout. When you come to that wind mill area, move the hay bales apart and use the key on that.
the door that's locked
There is no key to the dungeon. The small key open Mordred's underground laboratory, which has a back way into the dungeon. (see the related questions)
The key unlocks the entrance to Mordred's hidden underground laboratory. (You will find it under two bales of hay.)
The key from the secret society opens Mordred's secret underground lab, under the two hay bales near the mill farm.
The attic of the windmill doesn't have a key, only an ingenious secret. (see the releated question)
Go to the hay bales and push them apart. The key opens the hatch to Mordred's underground laboratory.
The key doesn't go to the saucer. It goes to Mordred's secret laboratory. (see related question)
You have to find the key. (see the related question below)
To push the haystacks in Astro-Knights, you have to get the small key from the fountain by pushing the crescent moon, the sun, the planet and the star. Then keep on pressing people until one gives you a small key. Run to the haystacks and STAND IN THE VERY MIDDLE OF THE TWO. The haystacks should naturally be separated at that time, and use your small key to enter the a small underground door.
you just click it.
The key opens the entrance to Mordred's secret underground lab. The entrance is between the two hay bales in the farm area near the mill. (see related question)