=> Same writers and probably same creative team too.
=> Both heavy on politics, might bore some people but if you're really invested, the dramas are both really satisfying
=> some supporting actors appear in both dramas (ex: Hong In Bang in 6fd is Seolwin/Mishil's lover in Queen Seon Deok + Alcheon in Queen Seon Deok is Bangwon's eldest brother in 6fd)
=> many memorable characters from main characters to supporting ones
=>great warriors/ swordfighters are depicted in both dramas

=> Both are Kim Tae Ri dramas
=> Fencing is featured in both dramas
=> The coach in Twenty Five Twenty One is also in Mr Sunshine as the mother of one of the female lead's suitors
=> I'm particularly using this comparison for Twenty Five Twenty One fans to watch Mr Sunshine as Mr Sunshine is underrated among international fans and new Kim Tae Ri fans should check it out.
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This may be weird, but I think the two shows have a few similarities. Both have some comedy to keep the audience entertained. Both shows' characters (or at least some characters in Vincenzo) also obsessed about looking for gold/treasure.

=> about a person who looks like the supposed real king/crown prince, the person was suddenly put in the high position of king/crown prince
=> the person that wasn't supposed to be king/crown prince proved himself/herself to be a good king/crown prince while he/she is on that position
=> the real supposed king/crown prince has a half brother, some of the plot revolves on the mother of that brother plotting against the crown prince/king (without her knowing that the crown prince/king is not the real one)
=> the main villain is an old official audiences will love to hate
=> BONUS: both lead characters (plus lead female in The Crowned Clown) in both dramas are played by former child actors with lots of acting experience so we can expect great acting from them

Lee Se Young plays a royal loved by a king in both dramas. Both Yeo Jin Goo and Lee Jun Ho are among the best actors playing Joseon kings.

> Both dramas by writer Kim Eun Sook. I'm recommending two shows she did, both of which are of soldiers, but the feeling of the two dramas are quite opposite. DOTS is romcom and Mr Sunshine is more melo.
> Both lead males are soldiers.
> The male lead in DOTS and the female lead in Mr Sunshine are both characters fiercely determined to protect their country
> The second lead couple' actors in DOTS appeared in Mr Sunshine as parents of the female lead.

Arthdal Part 1 is the first if the three parts released (all of which are Arthdal Season 1) while Arthdal Part 3 is the third part. There is also Arthdal Part 2, which must be watched between Arthdal Parts 1 and 3.

Arthdal Part 3 is the sequel of Arthdal 2. There is also Arthdal 1, the beginning part. But all these Arthdal Parts 1-3 are all part of Arthdal Chronicles Season 1.

Arthdal Part 1 is the prequel of Arthdal 2. There is also Arthdal 3, the third part. But all these Arthdal Parts 1-3 are all part of Arthdal Chronicles Season 1.

Both dramas are about the rise of Lee Bang Won. Difference is King of Tears is meant to be a more historical take while 6 Flying Dragons adds some interesting characters.

> Both dramas focus on court ladies
> The friendships of the different court ladies, their relationships with their higher ups, emphasis on how they should serve and belong to the king, is shown in both dramas
> TRIVIA: The female lead Lee Se Young is actually the child version of the rival female in Jewel in the Palace! Her characters in both dramas are both court ladies that are very talented. In Jewel in the Palace, she can cook since she was 3 years and is very knowledgeable in the kitchen compared to others. In Red Sleeve, she's good at writing and poetry.

Both dramas are about a princess who, due to circumstances, have to grow up outside the palace. For sometime in her life, she doesn’t know she’s a princess. She eventually returned to her kingdom and was a very good asset in helping/saving her people. Princess Deokman in Queen Seon Deok uses her wits, Princess Pyeonggang also does it plus she also fights some battles for her people.