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Necromancy for the Necromancer is the same as summoning for the Druid, resurrection for the Priest, etc. And it is possible to manage without them!
What? Is it getting difficult? Without necromancy? Well, there you go!
Conclusion: The Castle of Death doesn't rule!
I never said it ruled! Haven (and Nature, and Chaos) rules :) You can manage without necromancy, but who would want to. It's a really powerful skill. Why deny yourself it.
But if you've decided to give it up, magic won't suffer because of it. There's still the Hand of Death spell.