Although you find the captured robot invader in the dungeon, you cannot get to him from the library. Throw the switch to open the back gate to the cell, and return through Mordred's secret laboratory tunnel.
(see the related question)
There is no jail on Astro Knights Island. There is a castle dungeon, where an attacking alien robot is being held after his capture. (see the related question)
You get the alien "fuel rod" from the robot invader in the dungeon, and use it to power the saucer Excalibur, so you can go to space. (see related question)
There are no cubes on Astro Knights island. There are ice floes on the Ice Planet, that you jump across to avoid the robot fish.
Actually, Merlin has little to do on Astro Knights Island. He recovers the fuel rod from the dungeon when the robot self-destructs. And he battles Mordred at the very end of the island.
The green fuel rod on the Excalibur is broken and needs replacing. There is a rod inside the robot invader in the dungeon. (see related question)
you fight the robot on jungle planet
The robot invader only speaks binary. You can't get him out of the cell. You need something from him and you'll have to find another way in. (see related question)
One of the robot attackers was captured and put in the castle dungeon.
Things that Zap you on Poptropica : Lasers (Spy Island) Electrified Windows (Spy Island) Mini-Bots (Spy Island) Electric Fence (Diamond Mine, Nabooti Island) Electric Caterpillars (Jungle Planet, Astro Knights Island) Mordred's Robot (Astro Knights Island)
One of the robot attackers is in the castle dungeon. The alien ships attack you in outer space when you are near the Fire Planet and the Jungle Planet. Dodge their shots and shoot back, and land as soon as you can.
Throw the switch when you get the cheese. You can enter the dungeon from Mordred's underground laboratory once you have the robot owl. (see the related question)
When you awaken the robot owl Merlin, he flies out of Mordred's lab. In order to get him back, you have to feed him the robot mouse. (chomp)