Chapter 11 Walkthrough for Baldur's Gate: Siege of Dragonspear
This chapter begins when you arrive at Dragonspear Castle after the attack on the Coalition Camp. When you first arrive you'll have some dialogue where you're asked about what you learned when you went into the Dragonspear Castle Basement and which of the optional story objectives you completed. If you poisoned the Crusader's supplies or placed the Barrel of Bwoosh now is when it will pay off.
It's at this point in time where you'll be able to detonate the Barrel of Bwoosh! that you placed in the Underdark area of the Underground River. For those of you who didn't place the Barrel of Bwoosh or you didn't choose to detonate it when asked about it you'll have to circle around the Dragonspear Castle to the southeastern side where you'll find Phossey Dugdeep. Save her and her friends from the attacking enemies and then speak with her.
Phossey will place another Barrel of Bwoosh at the weak spot in the wall with all of the wooden planks which she'll then detonate for you to gain entry. As you could probably already guess our next objective is going to be gain control of the Dragonspear Castle Courtyard and take out all of the crusaders within.
During your battle in the courtyard when most of the enemies have been defeated you'll be interrupted by Ashatiel and her gang. Ashatiel proposes a one on one duel with your main character and promises to surrender if you win. She is telling the truth, however, if you choose to accept these terms you'll lose the EXP and loot of her entire squad.
Once Ashatiel has fallen return to the main area of the outer courtyard and approach the large doors leading to the inner courtyard. This will trigger a scene with Caelar where she stands on the ramparts overtop the gate leading into the courtyard. Again, whatever dialogue you choose doesn't alter the events. When Caelar leaves Phossey blows up the door and then you pursue Caelar into the castle.
If any of your characters died during the attack on Dragonspear Castle you can speak with Dosia before going inside the castle. She'll resurrect any of your characters that need it as well as provide you any other priestly needs. When you're ready to proceed head into the castle. On this floor you'll find some loot that you can collect as well as the vendor Belegarm.
You should take this time to prepare your characters as much as possible for what is to come. Loot whatever you need from the courtyard where the battles took place and sell whatever you don't plan to use. Stock up on items that you'll need to survive the upcoming trials and proceed to the large door in the basement that has Bence Duncan standing outside of it.
When you enter this room and proceed to the next chapter of the game there will be no turning back. Make sure your characters are well rested and fully stocked for what's to come; we'll be visiting Avernus and we're going to be fighting a lot of demons and a lot of powerful demons.
Tip: If you stand outside of the portal to Avernus without entering it demons still start spawning in the room. You can very easily farm EXP this way since demons will spawn indefinitely (as far as I am aware) and they're worth 4000 - 6000 EXP each.