However, I was taken aback and amazed at all the choices this segment in the game offers you, so I started making a list to keep track of it.
The longer it went, the more I felt like it should turn into a walkthrough for the specific part, with most of the choices covered in a not too detailed, spoileriffic way.
Those who are interested can read my findings and plan whatever route/choices/options they want to go/take.
For sake of cleanliness, I will post the walkthrough in spoilers.
Arriving at the Castle you get two options:
Option 1) Choose to charge
This leads to battle with six undead and gives you an extra choice when you confront the Vampires for the first time, provided that you have recruited Mesphit.
After the battle it leads to a choice between 4 walls. One of them leads out, ending all choices – you can keep choosing until you find freedom.
North – Item.
East - Battle.
West - Battle.
South – Freedom.
Option 2) Choose to sneak your way
This option leads to the party dividing into two teams of Myrth or Apolimesho. You have to choose one, but there is no real difference in choosing either except for their individual abilites/unique lines.
Which party members you bring with you affects certain lines/events, but none of which prevent you from getting items.
Then you will begin navigating the Castle/Catacombs.
Note: Some of the choices represented here will not be available depending on the party members you brought with you - i.e. if you have Dora, crawling through the hole will not be selectable as she will check it for you etc.
Note also that no matter which option you choose, you always progress a little further with the navigation - even if said option tells you it leads to a dead end.
Lastly, some choices lead to reaching the goal faster, as in going the right way. For example, following the sound of water, choosing to go forward towards the sound of enemies or going through the hole. Thanks to Aleema for this information.
It is entirely possible to get all three items and no encounters, if one so chooses, however.
If you end at hole
Choose either option, none leads to any item or battle.
If you end up at place where you choose 4 directions
North leads to battle.
East leads to battle.
South leads to next scenario.
West leads to item/dead end.
If you hear crunching and monsters are ahead
Going forward leads to monsters (duh!).
Going back leads to a new scenario
If you get to dig
Digging the ceiling makes it collapse on you, triggering funny responses from some party members, but nothing really happens.
Digging the walls leads you onward.
Digging the ground leads to item.
Going straight leads to battle.
Going left leads to item.
Going right leads to next choice.
Choose either, they both lead onwards
Left or right tunnel with/without bones
Left leads onwards
Right leads to battle
Whether you chose to charge the castle or navigate it, the next thing that happens is that you meet Morte.
After a brief introduction she gives you these choices:
Choosing to kill her
You battle her and her minions and do not receive any new quest. She fights like a mixture between Hero Class (resurrection) and Chambara (Necromancy, row damage). After the fight you get the achievement for ‘Killing Morte’.
Then you can proceed onwards to meet the Vampires.
If you agree to her terms
You get a new quest from her, about killing the Death Knight, Zeal. The quest takes you to the Orcish Wastes, where you can choose to answer “Yes” or “No.”
The first option lets you fight in the Orcish Arena and also introduces you to Zeal afterwards. You skip fighting cultists this way.
The second option lets you battle the cultists first, before entering the Orcish Arena - For fighting and besting the arena, you get the associated achievement.
You then learn that Zeal is in the Vork Lair (Previously empty when searched).
However, when you find him, you do not have to kill him (See Zeal part).
If you choose to leave
You go on and can choose either option above later (Provided that Morte is still alive, see the Vampires quests further down).
Meeting the Vampires:
After meeting the ‘man of the house’, you get two or three choices, depending on how you approached the castle.
Choose to get the diamond
You get a quest for going to Hammerhand and finding a diamond (See Hammerhand Part). You can still kill the vampires later and get associated achievement.
Give them Mesphit
This one is only available if you charged through the front door.
This option leads to Mesphit's affection towards you to decrease by 1 and leads directly to N'Mar Family bossfight, after which the vampires are dead and you get the associated achievement, but cannot receive their other quests, obviously.
This leads to fight against the man of the house Zachary + 2 Gargoyles. Afterwards, it leads to the boss fight mentioned above, N'Mar Family and after besting it the associated achievement. You obviously cannot gain any further quests from them.
Note: This option lets you get TWO Zachary Wedding Rings, if you want.
You get two choices here.
1) You can either pay 2,000 gold, if you have them and be done with the Vampires’ quest – However, you miss out on an achievement, look at next paragraph.
2) Or you can refuse and go to the Diamond Mines yourself, where tough battles await you – When you get out, you get the ‘Escaped the Diamond Mines’ achievement and get the diamond, so you can return to the Vampires and finish the quest.
If you have trouble with the Diamond Mines, I have a quick guide somewhere on this forum.
Meeting the Vampires 2nd Round:
When you return with the diamond they let the Amazons go and you fulfil your quest of saving the scouts.
However, if you return to Castle of N’Mar, they get annoyed at you.
If you chose to leave Morte when you first met her or accepted her quest, you are presented with three choices:
Accepting their proposal
They want you to kill Morte as she is annoying them – and you get this as a new quest.
Upon fulfilling this quest they are now friendly towards you and you get three choices.
If you talk to them, they hint at the fact that you can befriend/help Zeal.
You can STILL kill the vampires after this and get the associated achievement.
Note: Doing this will lock you out of completion for Morte’s quest, if you accepted it.
Refusing their proposal
They let you go and you can return and decide later.
Kill the vampires
Leaves to the boss fight with the family N'Mar and you can obtain the approriate achievement for killing them.
Zeal, the Death Knight:
Whether you find him on your own after slaying both Morte and the Vampires, not doing any of their quests or by following Morte’s lead – he presents you with an option of ‘helping’ him or fighting him.
If you fight him and Morte is NOT dead ( See Meeting the Vampires 2nd Round):
You get the King Slayer achievement.
It also lets Sauzer be ‘infected’ with Zeal’s spirit.
When you return to Morte, you finish her quest and then get some choices.
The top two sound different, but both lead up to your party fighting Morte and her minions, as she wants the now possessed Sauzer.
CHoosing to kill her, surprisingly, also lets you fight her and her minions.
You can also choose to leave and nothing happens
No matter what outcome there is here, do know that Sauzer is still possessed by Zeal’s spirit and this will have a bearing later and you cannot get the "Help Zeal" achievement in this playthrough. You'll have to reload, if you wish to do that.
If you allow him to live
You can ask him some questions and might decide that Morte is playing you.
You can then return and she gives you the option of leaving (Kill Zeal this time around if you want) or you can get to kill her.
If you do this, her associated quest with Zeal will fail and you will not gain the 500 XP for all party members.
You will be able to gain the achievement “Help Zeal” later on in the game, if you slay the vampires and report back in with Zeal.
To sum up:
If you want the most items/EXP with the least amount of loss -
Choose to find another way into the castle, find all three items and choose as many monster encounters as possible.
Then meet Morte, accept her quest. Accept the Vampires quest. Go to Hammerhand, refuse to pay and complete the Diamond Mines. Return to the vampires and accept their second quest.
The go to the Orcish Wastes, choose to fight in the arena. Get to know Zeal’s whereabouts and meet up with him. Fight him.
Return to Morte and get quest completed. After she’s dead, return to the Vampires and get the quest XP for killing Morte. Then kill the vampires.
For quickest route -
Choose to barge into the castle. Get the item and then choose the right wall to get out of the acid pit. Kill Morte right away. Kill the Vampires when you meet them. Done.
You will have to find Zeal yourself this way.
Getting the achievements –
For getting the most achievements in one go/playthrough, do everything as described in ‘getting the most items/XP’, but save before killing Zeal.
Then kill him and do as described above in the paragraphs. After the achievements, reload and let Zeal live.
You can kill Morte and the Vampires again if you so choose.
You will get ‘Help Zeal’ achievement later on.
Note: Doing this causes you to fail Morte’s quest, if you accepted it (end thus letting you miss out on 500XP)
Any questions or comments or corrections? Just make a post and I'll add with due respects.
Also, in case you're wondering... Yes, it took quite a while to try ALL choices. Don't try it. Seriously. You'll die. I just barely survived.
EDIT: Corrected a few typos and added a bit more on some paragraphs to be more concise.