Spellhold Overview - Baldur's Gate 2
Spellhold is where the Cowled Wizards are holding Imoen and the main dungeon that you will be visiting during Chapter 4 of the main story. To gain access to Spellhold you'll have to either be admitted as a patient or you'll have to defeat Perth in Brynnlaw and loot the Wardstone for the Asylum off of his body. For more information about either of these things check out Chapter 4 of my Walkthrough.
This page covers the first 2 floors of Spellhold which include the "residential" area as well as the torture chambers that Irenicus uses in the cutscenes. When you first arrive at Spellhold much of these 2 areas will be off limits to you, it won't be until after you've finished the Spellhold Dungeon and the Spellhold Dungeon Tests that you will have full access to both of these areas.
When you enter Spellhold for the first time this is the map you will find yourself in. The northeastern 2 rooms will be inaccessible and you will have to follow Irenicus around the area on a tour before finally talking to Imoen and being captured. It isn't until you've completed the Spellhold Dungeon and forced Irenicus to flee this area that you'll gain access to the northeast portion.
Jon's Key & Horn: In this room you'll find the Horn of Silence as well as Jon's Key which will open the door leading to the Underdark Portal.
Staff & Journal: You'll find a Staff of Thunder and Lightning +2 inside of this room as well as Jon Irenicus' Journal first and second halves. Neither of these are required to advance the story, they'll just give you some more backstory about what is going on and why.
After you complete the Spellhold Tests and get your Judgment you'll find yourself on this map. You should avoid the middle section since this is where Irenicus is found and instead visit the first floor instead to advance the story. In order to win the fight with Irenicus you'll have to unleash all of the prisoners on him at the same time you fight him.
The only other 2 notable things on this map are the portal to the Underdark in the southeast and the journal entry that you can find in the northeast. I'd recommend against going through this Underdark portal as you'll miss out on a whole map with a decent amount of EXP. For a full explanation of why check out Chapter 4 of my Walkthrough.
Irenicus: During your visit to Spellhold during Chapter 4 this is where your confrontation with Irenicus will be.
Irenicus Journal #1: Here you will find a portion of Irenicus' Journal which will give you some details about his motives and what he wants the Child of Bhaal for.
Continue to Spellhold Dungeon Overview
Continue to Spellhold Dungeon Tests Overview