It is a "brass plaque", on the fountain in the village. The same place where you got the gold coin to enter the museum. Once you have the Cosmic Symbols card, you can open the secret door behind the plaque.
you can get the coin in astro knights island on the fountain
to get the gold coin on astro knights you go on to the rocket ship fountin. at some point you will see a shimer. and that's were the gold coin is. hope that helps!
There is a gold coin outside in the fountain.
there is not a second gold coin
The gold coin in Astro-knights is in the fountain. You will see something glittering in the water, and if you pick it up, it will be the coin. Use it to pay your admission to the Mordred museum.
it's in the fountain near the building.
Jump up on the fountain (a gold coin is about halfway up on the right).
There is a gold coin in the fountain outside. Go in and get the Library Slip from the stack of books.
The gold coin is in the fountain next to the Mordred Museum (you can see it sparkling).On the fountain, second level to the right.
This is the first item on the island. Go to the Mordred Musseum and spend it. Get the library slip and go to the castle library.
In the fountain
There is one in the middle (upper part) of the spaceship fountain next to the Mordred Museum.