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On the other hand, some L3/L4 sites can give you a really powerful ritual discount or some nice units (recruitables or free summons).

Astral everywhere (non-ocean) is important for the potential units and 50% conj discount. Blood nations should Bowl of Blood in swamps and forests first.

Here are the really nice sites (imo):

Mountain:
E3 for Conjurer's Cave (30% conj discount)

All except water:
S3/S4 for Libary of Time/Grey Tower (Shadow Seers)
(S4 everywhere can also find the Strange House, I think it might have a void gate, but I've never found it.)
S3 for Ultimate Gateway (50% conj discount)
N3 for Oak of Ages (50% enchant discount)

Forest:
B3 for Blood Rock (30% blood discount)

Swamp:
B3 for Summoning Circle (60% blood discount)

If you are planning on a lot of Gift of Reason and Beckoning:
All except water:
D4 for Crown of Darkness (40% thaum discount)