In the fountain. you have to find the password first.
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.
You have to power it up with manure which you have to get from the farmer near the secret hatch to Mordred's Hideout
When you have all three of the Knights together, you will travel to the secret hideout of the Binary Bard in the Asteroid Belt. (Secret location : X-11 Y-82)
If you are talking about the one in astro knights island, just push on the part that is blocking the sort of hallway and it will eventually cave in.
There's a lock under the hay in Astro Knights
you push through
It opens Mordred's secret underground hideout which is beneath the hay bales near the farm. Separate them to reach the locked hatch. (see related question)
Mordreds secret hideout is between the two hay stacks, but first you have to get a key to go through.
you push the wall until it brakes
go to the secret door
The secret room between the hay bales!
There is a secret society inside the fountain in the Town Square. You get the code from the creepy girl in the Mill.