Yeah, Spell of Making has been my go to VC, so it's probably a little too easy as it stands. Moving it down the tech tree might be a start.
In one of my recent playtests, I saw three empires/kingdoms start building towers before I had the relevant tech, some of which had the third tower online before I built my first tower. I was able to get all four towers built and cast Spell Of Making before they could, which I'm still scratching my head about. Is the AI not 'seeing' this VC correctly?
I sort of agree with the splitting up the towers between several techs idea, or at least the requirements for spell of making.
The three towers, by the nature of their bonuses, are inherently magical. So they probably should remain in the magic tree, OR, be merged into a single tower with upgrades: i.e. bonuses to 2 elements, upgrade 3 elements, and then upgrade to 4 elements for the 'Full Monty' Tower. As there are 4 towers, 3 prerequisite to the fourth, I actually like the idea of having 4 techs for the 4 towers.
I do like the multiple towers though, but for a different reason than you might think. Their footprint when placing them makes them nice for expanding your city borders next to an improvment (shard, stables, etc.), so they can be incorporated within the city borders.
The other two 'structures/requirements' for Spell of Making could be put in other trees. The question is: 'What non-magical requirements make sense for a Spell of Making?"
A suggestion might be 4 techs in Magic (one for each element) with the final required tower to merge their effects requiring some civ tech, perhaps involving a combination of crystal and metalworking or somesuch. I can't think of a good reason to include a warfare tech, but perhaps others here might have thoughs on this/which tech might be relevant to such a pursuit.
This is definitely a VC that needs further discussion and tweaking.