Haven't quite gotten around to the Necromancer's castle, but I'm not looking forward to it as I'm playing on Hard.
I am level 24 now - probably 22 Attack, 22 Defence, and 7 Intellect. (Paladin).
I've still got to finish the Gremlin castle before cleaning out the last of the Land of the Dead. I've been to Demonis but not fought anything yet. Leadership is just below 20,000.
I'm pretty sure Demonis is harder. That will be next....