Rooftop Prince

Both male protagonist are from past, it involves time travel, love and mystery

Similarities:
=> Both are stories of a romance between a woman who is supposed to be a crown princess and a man destined to be king
=> A 12 year old Kim So Hyun gives a very excellent performance as the young version of the woman who usurps the crown princess position. (In a way she's actually the one who played the usurpers, not the adult actresses, because the takeover of the crown princess position from the real crown princess is done by her character in her teen years).
=> We see the male lead (or rather the young version of the male lead in Moon Sun) attracted to a girl with a butterfly in the first episode.
=> The male lead (him or his doppelganger in Rooftop Prince) has a close relative who can be a threat for high position he has (the king position in Moon Sun, being the heir of the company in Rooftop Prince)
=> Lee Tae Ri/Lee Min Ho actor born 1993 gave excellent performances in both dramas. Both his characters in a drama is a very smart young man who helps one of the main characters.

Both are about a crown prince who loses his crown princess at the begining

This drama is just...WOW!! Way better ending than Rooftop Prince and the acting is even more fanstastic! Better OSTs and everything. Plus KIM SOO HYUN *(drools)*

Both male protagonists have lost their Crown Princess at the beginning of the drama, there is no time travel, but it is an historical romance

Accidental time travel, love, and finding who you really are, theses are the themes that make these dramas irresistible! But this time its reverse! What happens when a king ends up in the future on a girls rooftop? Well hopefully you have seen splash splash love this time its the king's turn to be the pauper!

It't just amazing, the well played role by the main cast, the twist plot which would keep you wondering and the humor. Its just a great combination.

Both leads are hilarious without even trying. Feel-good Dramas to the max!

The leads are goofy and a tiny bit awkward. The situation they stumble upon (soul/time switching) make them even more confused and hilarious.

both about time traveling rooftop from past to future and Dr.Jin from future to past :))

Both saeguk, both with Micky/Park Yoochun in the leads, in both he is a known, rich person

Crown Prince Yi Gak finds that he has been transported from the Joseon Dynasty to modern-day Seoul. He meets Hong Se Na, who bears a striking resemblance to his dead wife, and is determined to solve the 300-year-old mystery of her death. With his own resemblance to the CEO’s grandson at the company where Se Na works, Gak assumes the identity of the young man to stay close to her. Can the Crown Prince navigate a modern corporation to find clues to his own time period?

Both shows have one of the main characters slip into a coma, and somebody who looks just like him takes his place and wins the girl. (In Rooftop Prince, they are actually the same person. It's a time travel drama, and the one in the coma is the main character's reincarnation.)

If you enjoy watching Han Ji Min falling in love with a crown prince in Rooftop Prince, Yi San is also another story where we can see her fall in love with a crown prince.
There is a great genuine chemistry between the crown prince and Han Ji Min's character despite the fact that her romance with the crown prince seems impossible.
Be mindful that Rooftop Prince fans might not like the politics of Yisan or Yisan fans might not like the modern era in Rooftop prince, but both dramas are worth watching to see a blossoming romance between a crown prince and Han Ji Min's character

I just realized these two dramas have similar vibes.
- Both are time-travel sageuk dramas
- Joseon Dynasty
- Beautiful Love Story and amazing lead couple
- Both male leads are tricked by second female leads and they thought SFLs are their childhood first loves but actually FLs are.