In Philly we just saw Episode 36 where Tang is laying siege to Pyongyang. Looks like The city may fall soon. This got me thinking that (unlike many other historical dramas) we haven't seen any evidence of a crown prince of Goguryo yet. Doesn't King Bo Jang have any sons or nephews? In an early episode it was mentioned that he had a number of daughters. But what about male heirs? Or did Yeon Gaesoman get rid of them all when he purged the previous king? Since we only ever see him conferring with Sukkyung, does this imply she's the nearest relative?
On a totally unrelated subject, I noticed that the Tang/Kitan army, while battling Pyongyang, has found time to dig out and build a nice underground dungeon in their camp (DJY was thrown in there during the "peace" talk with Bu Kiwon). How farfetched is this? I do seem remember another undergroung dungeon in the Kitan settlement early in the series when DYJ and Sue Rengi were imprisoned by the Kitans. They were definitely underground with stairways.