It would seem to me the easiest solution is use the form bonus already in the system. A twenty in Terram would give you a +4 to "earth related skills", which is quite nice.
Which implies that the ex Misc tribal mage with a Terram 20 and the Jerbiton architectural apprentice with Terram 20 and Architecture 7 (towers) would produce the same product - which is not satisfying.
"Remembering" presumes some pre-existing knowledge - and if a player forgot about it (or was ignorant of it), with all the modern education, communication, constant local examples and internet available, what is so incomprehensible about a medieval mind committing the same errour?
Pointless? Perfectly useful on flat bedrock. The SG questioned it once, and the player insisted. There is an unspoken rule re players who ask for more than the usual offering (which the canon spell represents) - the Rule of Toyota - "You ask for it, you got it."
No one is disagreeing with that. The distinction is the final quality, the reliability, and the suitability of the item created for the intended purpose. A mage can choose to create a lance to batter down a door, or a battering ram to battle a dragon - but each probably would be better suited to the other purpose. Nothing in the Arts necessarily tells them what the best tool for the job is, and without the "Common Sense" virtue, players are responsible for their own mistakes. Meanwhile, a mage with "weaponsmith 2 (lances)" might reasonably be able to envision and thus produce a slightly more specific and superior tool for the purpose, rather than a Platonically generic (if also "Platonically perfect") one.
Sure, the Jerbiton might unaided come up with a different looking solution to the ex Misc, because the Jerbiton magus is familiar with more mundane architectural features.
However, if that solution is implemented with Terram magic, both the Jerbiton and Ex Misc would be equally good at producing it, because they are equally good at Terram magic. So, if the Ex Misc magus saw what the Jerbiton magus had done, he could reproduce it --- even if he didn't understand the mundane mechanics behind what was being done. Alternatively, if an Ex Misc happened to, say, travel to Florence and see some well made mundane towers, he would have no trouble reproducing those Hermetically (assuming his Creo Terram was up to the job). Just because the Ex Misc doesn't understand how the job is done mundanely doesn't prevent him doing the job with magic.
Also, you seem to be presupposing that the mundane way of building a tower is better than the Hermetic way. I don't think that this has to be true. And, generally, ArM5 assumes that Hermetic magic is better than mundane technology. If the Hermetic way of building a tower is better, it really doesn't matter what the Jerbiton knows about mundane ways of building towers.
The character, has apparently extensively studied Terram magic (and probably has a high(ish) Intelligence Score). So what the character does should be mostly clever things with Terram. The fact that the player doesn't really know anything about Terram, and has apparently made a silly mistake, shouldn't be an issue. A player botch shouldn't translate to a character botch.
And if you believe that Terram magic includes a study of foundations for buildings and other such mundanity, then you're absolutely correct for your saga. Otoh, it appears that this one did not believe that - and so he was absolutely correct in his ruling for his.
I'm not arguing that Terram is the study of foundations and other mundane knowledge.
I'm arguing that Terram is the study of how to manipulate rock and so forth using Hermetic magic. A magus doesn't also need to know how to manipulate rock mundanely to make a good job of making, say, a building with Terram magic.
The character can be entirely ignorant about the purpose and existence of foundations. However, his knowledge of Terram magic means that he knows how to make a good building with Terram magic. So, if a good building requires foundations, then that's what he will build with Terram magic (unless he botches or something).
A good rule is that you can obtain better info with Finesse that is that connect the relationship between Magical capacities and mundane effects, with bonus with your mundane experieces, lore and knowledge.
If you in the creation fo the spell are thinking in make somethig you can have spend time "practicing" the related, then you can figure how is, ask something beetwenn lab time and lab time and get experince points from exposition to the related ability. In this Case can be: Area Lore or cave Lore (Maybe Natural Lore too), Philosophia (geologic from Natural Philosophia), Masonery and so on...
Finesse is always unattached to other relevant Abilities - the conclusion to draw is that the rules intend that single Finesse roll to replace every aspect of the magical creation. Craft (mason) is it's own Ability, so Rego magic should be able to approximate it with Finesse.
The real trouble I see with the idea of carving out caves with ReTe is that it's two+ seasons of work to do a single (above-ground) tower - one can extrapolate that to perhaps a season per cave chamber or so? Meanwhile, the Finesse roll to pull it off a day's worth of masonry work is: Average difficulty (9) + Rego Craft Magic adjustment (3) = 12. That's for a day's worth of work, meaning that you'll have to pull of tons of these rolls... or increase the difficulty by +3 to do a month at once, or +6 to do a season at once. Reliably trying to hit a 15 Finesse is difficult; an 18 is nearly impossible.
This goes back to an earlier comment about Craft Magic being too difficult. Personally, I don't think it's too difficult, it's only merely difficult. More to the point, a magus with Affinity, Puissance, and Cautious with Finesse (specialty: Craft Magic) and an extreme score and DEX (+3 or higher) is the sort of rare and astounding wonder-worker that they should be. Given how Finesse works universally (rather than just being entirely replaced with +magnitudes for "complexity"), it has to be so; otherwise, you end up with magi being able to willy-nilly whip up whatever they need in high-fantasy fashion.
That said, I'm personally totally okay with the generic +magnitude for complexity to build-in high Finesse results into formulaic magic.
Also, a CrTe spell to create an entirely above-ground building. Hm. Luke 6:46-49 says this is dumb and Eph 2:20 is pretty clear on the metaphoric necessity of foundations, so I imagine anyone that speaks Latin well enough to be a magi would at least be aware of that. Also, in accordance with Platonic Ideals the spell would probably create some kind of radically-tapered conical structure with thicker base walls designed to sink into the soil and self-stabilize, since that's the only way I can imagine it working.
IMS I use a mixed approach. If you use Rego, Muto, or Perdo - you better know what you're doing to some degree. If you use Creo, the connection to the realm of forms makes up for many gaps in knowledge. That is, CrTe create a mundane clock requires no prior knowledge of clockwork. Creating a specific kind of clock might be a finesse role. Similarly, creating a tower from the realm of forms would put the foundation into the ground because that's the way a 'platonic' tower stays up.
Also, if you came to me and really wanted your foundation to sit above the ground - I'd give you an inkling that this doesn't work in general and the reason you 'have an inkling' is based on your connection to the realm of forms - Creo, not Terram. I'd let you do it with a finesse role but the 'mundane' consequences of an improper foundation would apply. If you wanted to do it and have it 'stay up', it'd require some knowledge of building and the suitability of different grounds to support such a structure. It'd still be a very tricky role in the end though.
Personally I think it’s bad form for a player or a SG to go all the way through the creation process of an effect then spring a gotcha. While the spell or whatever is being designed everyone involved should weigh in on their interpretations of how the effect will work and how it will be used.
When a story guide nerfs a spell because of something a player didn’t think of or a player gets a minor effect approved and proceeds to exploit it in ways they never mentioned in design, that’s not fun storytelling it’s one person telling another "I'm smarter then you".
Any and all of the Forms and Techniques can be argued to derive from the platonic realm of forms. Perdo removes things from their perfected state, Muto changes them to incorporate parts of other forms, and Rego controls them according to their natural capabilities. I doubt anyone'd require the character to be an anatomist and butcher before allowing a MuCo(An) spell to transform them into a wolf, after all. Animal represents understanding of how animals interact with magic, just as Corpus relates to the understanding of how the body does so. This is not necessarily academic knowledge ("His liver is damaged" as opposed to "The balance of humours has shifted, resulting in damage to his liver, in accordance with his diet and profession, and may be rectified by treatment with tincture of asafoetida") but it is complete in itself. A knowledge of Terram does not include knowledge of how to construct earthworks by mundane means, which requires rollers, teams of men, careful arrangement of the layers of stone and gravel in the right order and many other things. A magus with such knowledge does, however, know how to construct earthworks by magic and what earthworks are. In much the same way, he does not know how to fold steel to make a damascene'd sword, but he does know about the structure of a sword and the difference between types of steel. He probably couldn't tell you what types of stone are mined where, save for those with unique properties, but he does know what kinds of stone exist and what their virtues are. It is the difference between a foreman and a workman - the foreman does not need to know how each part of each job on the building site is best done; he needs to know what to instruct his workmen to do in conjunction with each other. A mundane worker is a workman; a magus commands magic to do the work for him.