The underground lab left by Mordred is under the hay bales on the farm near the Mill. To enter the lab, push the hay bales apart and use the key. To get the key :
There are no sewers in Arturus. The locked hatch is Mordred's secret underground lab. (see related question for the key)
In the underground lab of Mordred, there is a mud wall blocking your path. Push hard and it will fall down.
The wall that blocks the passage on the left side of Mordred's underground lab is NOT rock, but dirt. Push against it and it will fall down.
In Mordred's underground lab, the wall is just dirt. Keep pushing and it will collapse. This is the back way to the dungeon.
The key from the secret society opens Mordred's secret underground lab, under the two hay bales near the mill farm.
You don't. Once you have gotten the cheese, and pulled the wall switch, that is all you can do there. You will have to enter the dungeon from Mordred's underground lab, through the tunnel he dug to escape. (see the related question)
In Mordred's underground lab (beneath the hay bales), there is a wall to the left that blocks you off. It isn't rock, it's dirt, and you can push right through it.
The hay bales are OUTSIDE by the farm, and between them is the locked entrance to Mordred's underground lab. (see related question)
The haystacks (hay bales) are on the farm near the windmill. If you push them apart, you will find a hidden door that leads to Mordred's underground lab. (see relaated question)
There is no wishing well on Astro Knights island. There are :The spaceship fountain - there is a gold coin in the water, and if you take the secret message from the castle to the mill, the girl there will give you the password to open the secret passage under the fountain.The underground lab - Mordred's secret lab is hidden under the two bales of hay on the farm, and you will need the key from the secret room under the fountain.
When you enter Mordred's underground lab, feed the mouse to the owl, then push through the dirt wall to reach the path back to the dungeon cell. (see related question)
The cell is only "blocked" by the bars on the dungeon side. When you throw the switch, it opens the back gate, which leads to Mordred's underground lab. (see related question)