Xaart
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1. I chose the Skull as my starting bonus instead of the Shield, which gives +2 to Knowledge (+20 to mana).
2. With 40 mana, it's more comfortable to use Meteor Shower on 2-3 stacks of units (screenshot 1).
3. By level 5, you can easily take Expert Earth and Expert Necromancy (screenshot 2).
4. On day 7, I was there, there was no wall there... (maybe you should disable HD+).
Added 1 minute ago
Screenshot 3 - I can't get to the Spearman Tower - it's not an invisible wall, but neutral creatures in the way.
I'm playing the regular version, the HD mod didn't install because of the 8.