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250 gold still makes a big difference. But what makes Sinithar so great? He uses less mana... so what? Even Irbeth's ritual provides a significant amount of extra mana, so what's the difference?
New here? I'm Roha — want a quick tour?
Kifira is good at the start, and extra money never hurts anyone. After all, she comes with the Treasury skill. The mine guards are good at clearing out enemies (if the guards aren't ranged units).
But what if you need to go somewhere and eliminate someone, you have limited mana, and the next day they will capture your castle? You need Sinistar.
You can buy a second hero, and you'll get a decent army.
Those six to nine Minotaurs can also help.