Yeah, I love the bonuses the various towers provide, so I'd build them Spell of Victory or not.
A quick recap of a suggestion I made earlier:
Spell Of Victory casting cost should scale based on the number of Elemental Shards you control versus the total number on the map. So, say if you control 50% of all the shards, your cost would be 500. If you only control 25%, your cost would be 1000. Or some other algorithm that lowers your casting cost if you control more shards.
Note that I bumped the casting cost. If you can come up with 500 Mana for the Plague (my usual course when the event comes up, this event comes up in almost every one of my playtest games BTW), coming up with 500-1000 Mana for a Spell Of Victory is appropriate. We've had several stories now of mana pools in the thousands, and as this is a VICTORY CONDITION, yeah, you should have to work for this spell.
And, yeah, the tech requirement needs reworking, so that the technology to cast this is achieved later in the tech tree. I love the towers, though, so when they appear is fine. But perhaps the actual Spell Of Making isn't available when tower #4 shows up in the tech tree, but in some later tech, say Ereog's Journal...