Duo's Dangerous Palace

Duo's Dangerous Palace is a scrapped course which is extremely rare and can only be accessed in an extremely personalised copy or by modifying the code of an ordinary copy. It is found on Floor 13D at the end of the Eyerok the Quick Apparition's hallway behind the Unlockable Door, which can only be opened by collecting the Bronze Ghoul Medal in Juicy, Juicy Rhubarb.

Missions

 * 1) "LOOKS LIKE YOU MISSED YOUR LESSON"
 * 2) "DUO'S 14 VIOLET COINS"
 * 3) "HIDDEN SLIDE IN DUO'S NEST"
 * 4) "SPANISH OR VANISH"
 * 5) "KOMM HOL DIR EINEN STERN"
 * 6) "HO UN'ALTRA STELLA"
 * 7) "HIDING AMONG THE BOOKS"
 * 8) "YOU BROKE YOUR STREAK, I'LL BREAK YOUR LEGS (Yet to be collected by a player)"
 * 9) "PROVOCATIONEM AD ULTIMUM SIDUS"
 * 10) "100 COIN STAR (Presumably)"
 * 11) "200 COIN STAR (Definitely)"
 * 12) "500 COIN STAR (Most Certainly)"

Looks Like You Missed Your Lesson
At the top of Duo's Nest, there is a star where Duo normally sits.

Duo's 14 Violet Coins
Like almost all 8 Red Coin missions, collecting all the stars makes the star appear. Instead of 8 Red Coins however, there are 14 Violet Coins.

Hidden Slide in Duo's Nest
In Duo's Nest, there is a pipe which leads to Duo's Secret Slide. At the end of the slide, there is a star.

Spanish or Vanish
Immediately after entering the level, Duo will say 'Looks like you missed your Spanish lesson, you know what that means. SPANISH OR VANISH!', swoop down to the ground and begin attacking Mario with books. After ground-pounding on Duo, he will explode and a star will appear. This mission and boss battle also appear in the course Hyper-Realistic Hill.

Komm Hol Dir Einen Stein
In this mission, Duo is now near the pile of books where he will give you three random questions about different languages. If you get one of the questions wrong, Duo will kill Mario. If you get all questions right, Duo will give you a star. These questions are quite easy.

Ho Un'altra Stella
Almost identical to the mission Komm Hol Dir Einen Stein, Duo will ask three questions of a higher difficulty. These can be quite hard if you didn't do your Duolingo lesson.

Hiding Among The Books
The star is in an Item Box hidden between the books in the corner of the level.

You Broke Your Streak, I'll Break Your Legs
Like the mission Spanish or Vanish, Duo will battle Mario and a star will appear after defeating him. However, the difficulty has been increased to the point that the boss is practically impossible to beat.

Provocationem Ad Ultimum Sidus
The same as Komm Hol Dir Einen Stein and Ho Un'altra Stella but the questions are almost impossible to answer if you didn't do your lesson.

Geography
The layout of Duo's Dangerous Palace is similar to a library. In one corner there is Duo's Nest which is on top of a tower of books; precise platforming is needed to reach the top. Depending on the mission, Duo might sit in his nest and will give good advice about learning languages. In another corner is a pile of books where sometimes sits. In the other corner is another, larger pile of books where a star is hidden. In the middle of the floor is a red carpet where boss battles take place.

Duo's Secret Slide
Duo's Secret Slide is quite similar to The Princess' Secret Slide, apart from the color of the slide is lime green and there is an alternate path which takes Mario to a room where Pokey's Desert Key (needed to access Sandy, Sandy Desert).

Trivia

 * The rumor that Duo, the mascot of Duolingo, murders people if they miss their language lesson actually originated from this level.