The Three Musketeers (2014)

Same setting, same characters. Both are about the Manchu invasions of Joseon. But unlike Three Musketeers, Cruel Palace is not an action drama, instead, it focuses more on the drama.

Both are inspired by Alexandre Dumas novels, showing that a great storyline remains great as a translation into another culture or time period! Yay for classic literature (even if it was pop lit at the time!)

He Fei's wife, Li Mu Zi, disappeared mysteriously during their wedding anniversary trip. While he was searching for Mu Zi, she appeared again out of nowhere, but he insists that the strange woman in front of him is not his wife. Lawyer Chen Mai gets involved in this bizarre case, and more mysteries slowly emerge.

- Both are set during the same time period although in The Three Musketeers Crown Prince Sohyun is a protagonist

If you like historical dramas with a lot of action and a slight comedy, these two are definitely your type.

not so similar but both have interesting touch to Crown Prince Sohyeon (who was ahead of his time, wanted to bring modern things in Korea).
While he is ML in 3 Musketeers, In The King: Eternal Monarch. Crown Prince Sohyeon, in a parallel universe, lived on to become king. His reign would cause a chain reaction that culminated in a modern constitutional monarchy.

both drama ask question, what would be if Prince Sohyeon became King as he should be.