Level 18 is a little on the low side for this quest. Actually, you do not need to rush any of the quests that give you an invitation to the castle. I did most of the quests in Rivertown before heading to Verdistis (didn't know it was a relatively small area), so Yarun's assassins were easy to deal with. I also did most of Verdistis and cleared the Rivertown sewers before heading to the castle. The area after that can be a little tough at lower levels.
I maxed Aura of Guarding and Spell Shield with purchased spell books. They came in handy for part of the game. Spell Shield is good protection against poison, since the damage comes off the shield and your character does not jerk around with reduced offensive and defensive capacities (this makes it easier for a melee character to handle the zombies around the cursed abbey, for example).
