Both are heavy political dramas and have a Crown Prince as ML. Both crown princes are very similar and want to be good kings who really care for people. Haechi has more action while Secret Door is more psychological. One thing to keep in mind is that the 2nd male lead in Secret Door is also the main character in Haechi. Both characters and dramas are based on true story. Obviously each drama tells its own version about personality of this prince (and later King).
