Principes are excellent. Beyond that, Pythium is about Astral magic and communion. That's my view anyway. Growth is probabably a good idea due to the old age problems.
Wouldn't you prefer Emerald Warriors over Principes?
They cost too much IMO, both resources and gold. Also, Principes have strategic move 2, Emerald Guards have 1.
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Great indebtedness does not make men grateful, but vengeful; and if a little charity is not forgotten, it turns into a gnawing worm.