Enter the castle, you need to use a sandwich to become "beefy" enough to break down the door. A sandwich can be obtained in Lava World or purchased from the Volcano Store. Use the sandwich to become "beefy", then rip off the metal door by repeatedly tapping /. You can come to the castle early but, without a sandwich, the Industrial Prince will simply mock you from the balcony above.
Once inside, you will meet two waves of Fencers and Industrialists. Then you will encounter three mechanical obstacles in immediate succession. The first obstacle is a row of cannons incessantly firing acid balls at you. Your safest bet is to block these projectiles with your shield ( / ) as you slowly advance forward; however, you can jump over these projectiles with a little practice to save some time. Proceed up the ramp and you will come to the second obstacle which is a narrow path with large weights on chains that rhythmically rise and fall in an attempt to smash you. Timing through this is fairly easy: quickly step underneath the next weight as it rises, and repeat. You won't get caught under the previous weight if you always move at your first opportunity. The third obstacle is a row of electric barriers that rhythmically flicker on and off. You have room between each barrier to stand safely, so take your time advancing through each barrier one at a time.
After passing the mechanical obstacles, you are drawn onto a moving elevator. You will be confronted with a handful of Fencers while the elevator continues to rise. Once you defeat all of them, the elevator will come to a halt and you can continue forward. Next, you will be confronted by two small waves of enemies, the second of which containing a beefy Brute that possesses an animal orb, the Cardinal. Defeat him to claim his Cardinal. The two boxes in that area have a fruit and a drumstick.
Finally, you'll meet the Industrial Machine boss. Stay on the dark gray floor in the center, as the red areas to the right and left will burn you. You will be confronted with two mechanical weapons at first.
Electric Posts:
Five posts will rise out of the center floor (one at a time in this order: top left, top right, bottom left, bottom right, center). The four corner posts shoot electric projectiles, fanning over a wide angle; the center post sends a shock throughout the entire floor surface when the meter charges up. Hack away at the posts while they are risen until each one is destroyed.
Mechanical Hand:
Mechanical Hand
The hand does not come out until after the second post recedes back. It attempts finger-walking directly at you, and will usually send you flying right over the red areas, where you'll take fire based damage over time, and will probably knock you down, allowing your character to be attacked again if he doesn't move quickly while standing up.
Hitting the hand causes it to jump away from you. To avoid being hit choose ranged attacks. Be aware that some attacks (like the Red Knight's splash attack) won't push the hand back and eventually it will hit you. Shoot arrows and projectile attacks to keep it at bay.
Elemental Turret:
A gun periodically lowers on the right side and pans up and down the playing field, firing projectiles at various intervals. The type of projectile changes upon each appearance of the gun, chosen from among three elements: Fire, Ice and Poison.
It's probably best to take out the electric posts first. After that, it's your choice whether you tackle the hand or the elemental gun first. The gun is pretty easy to anticipate and avoid, so you may opt to destroy the hand first.
Once these are conquered, the Industrial Prince will try to flee, but the Evil Wizard will take the Orange Princess with him and leave the Industrial Prince behind. You can now proceed to the right (the dangerous areas on the sides are now inactive). Grab the telescope to end the level. Before you grab it, you may want to bring Cardinal (or go back for him, if you didn't bring him to the fight) and walk through the back room of the balcony. He will fetch you the Pointy Sword. Knocking the king off of the castle doesn't have any other purpose other than for humorous reasons.
Weapons
Zigzag - Return to this level after acquiring the shovel. Near the beginning, after you fight the first wave of enemies, there is a place near the back wall where you can dig up this weapon.
Pointy Sword - If the Cardinal is equipped, he will bring you this weapon from the back area of the balcony at the end of the level.
Utility Blade - This weapon is found at the beginning of the level. Search behind the bushes outside the castle before entering the door.
This level may a reference to the game Teenage Mutant Ninja Turtles (other being the Alien Ship boss). Many of the obstacles which the players have to confront, and the overall design of the level, are very similar to the Technodrome level (stage 5) of that game. The Elemental Turret looks and behaves almost identical to the laser guns in that game. The electric barriers which turn on and off, and which you must carefully cross in between, also make an appearance. And the elevator ride looks almost identical too (although here it goes up, and in the "Turtles" game it goes down).
/
. You can come to the castle early but, without a sandwich, the Industrial Prince will simply mock you from the balcony above.
/ 
) as you slowly advance forward; however, you can jump over these projectiles with a little practice to save some time. Proceed up the ramp and you will come to the second obstacle which is a narrow path with large weights on chains that rhythmically rise and fall in an attempt to smash you. Timing through this is fairly easy: quickly step underneath the next weight as it rises, and repeat. You won't get caught under the previous weight if you always move at your first opportunity. The third obstacle is a row of electric barriers that rhythmically flicker on and off. You have room between each barrier to stand safely, so take your time advancing through each barrier one at a time.
