Both dramas are centered around the same premise and history and same 50 episodes. If you are more into something more mature, historical accurate and grounded check out Jeong Do Jeon.
For those into more to action, adventure, romance check out six flying dragons. Or do both, they are all very well made dramas.

Jeong do jeon is direct prequel to The great king, Sejong. And The great king, Sejong is direct sequel to Jeong do jeon. Highly recommended to watch Jeong do jeon first then the great king, sejong.

Both dramas staring Song Il Gook. Both rags to riches kind of story. Both characters has a loyal side kick from since childhood. Both have forbidden love with a princess.

both is about the main character's journey to become the King. Since both drama are produced by KBS, the way is tells it's story it's similar. It's also nice to see Kim Yeong Cheol and Kim Gab Soo, playing king and subordinate in both shows before our main character succeed his character as King. The Great King, Sejong mentioned about Goryeo a lot, to learn about the history you can watch Emperor Wang Gun to understand part of the story.

Granddaughter of Wang Gun so Emperor Wang Gun drama is kind of a prequel and that makes The Iron Empress a sequel. Plus both shows are produced by the same TV network so you will also find some of the casts are in both show.

Gwanggaeto name is referenced a few times in emperor Wang gun, if you are interested of what they are referring to, go watch the show. both shows produced by the same TV network, you will see some of supporting casts are in both shows.

Story about founding of a country, similar premise (mention of Goguryeo), same lead actor playing the lead title character of both shows. Also you will find some of the supporting casts are also both in these shows. If you are interested in history, the country Balhae is also reference/mentioned on emperor Wang gun.

Funny that these two related shows came out the same year, you will find some of the characters from this show are also in that show portrayed by different actors and telling almost the same story but from different perspective. If you enjoy this show, you will enjoy that show as well.

Both dramas' story are set in Unified Silla Dynasty, both main character from both shows are played by the same lead actor and about the same premise, merchant, sea, war. And this show has reference Jang Bo Go the main character of emperor of the sea a few times in emperor Wang gun and also directed by the same director.

The story take place about the same timeline as this show just that it is from Baekje or opposite perspective.

The movie's premise is same as the show, some of the characters from the show are also in this movie but portrayed differently

As you can see same characters, but just that for dong yi the story is a retelling through Consort Choi/dong yi perspective.