Where do you go when the guards won't let you in the dungeon on Astro Knights island?

Answer 1

In Mordred's Laboratory, push down the dirt wall to enter the tunnel that leads to the back of the dungeon cell. When you entered the dungeon through the library, throwing the switch opened the small back gate. You can enter the cell. But to get the green fuel rod, you will need to feed the mouse to Merlin, the mechanical owl. (Click Merlin and then the fuel rod)

Answer 2

Once you have thrown the switch and grabbed the cheese, you can't go back in through the library. It is just as well, because you can't open the cell. You can reach the opened back gate by going through Mordred's secret underground laboratory.

(see related question below)

Answer 3

When the guards won't let you into the dungeon on Astro Knights Island, you need to first find a way to distract them. Look around the area for items or clues that can help you create a diversion, such as ringing the bell or finding a disguise to sneak past them. Once you successfully distract the guards, you can gain access to the dungeon.