IMO witches get a huge bump from lvl 1 to lvl 2. Spawning 4 skellies at a time vs 3 is huge and 8 max vs 6 max. That being said witches are specific to certain armies where golems are used in just about every attack. So start with golems.
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royalty is the longest upgrade in the game, end of story.
I think you should do golem