Mish-gan
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That's the point – there are no unbeatable races. For each, you can find a counter. Gnomes can be adequately countered (with equal player skill) by Humans with a lot of cavalry and Mages with mini-arts. The rest need to rush, and this isn't an imbalance, but a variety of strategies. I think Necropolis is a very dangerous opponent (for Gnomes) only up to weeks 5-6; after that, runes and superiority in combat stats make themselves known, and the Necromancer's chances diminish with each week.
I agree with almost every word in your post!
I'm glad you actually looked at the obvious things!
The only thing I would change is Mages to Orcs; Orcs are very strong against Gnomes, especially with Stronghold. I'm currently working on a tactic against them; I have an idea about "runes of etherealness" and "readiness," but I probably won't be able to answer about its feasibility and effectiveness until the weekend.