Runic magic

I agree, and what's most unpleasant is that developing the Rune School hinders the development of the Castle and consumes a lot of resources. Also, Rune magic isn't that overpowered; in my opinion, it's better to upgrade the Guild of Mages, and I use Rune magic quite rarely.

Mana is a resource that tends to run out, and Gnomes have no way to replenish it (we're excluding Rune cooldown, since you said you don't actively use Runes). Therefore, it's better to spend resources on resurrection than on 15 mana (because the main battles usually happen when the Castle is more or less built, and resources are not as crucial). And so on.