A curious situation has come up, making me wonder how well Magi consider ventilation when designing their ritual spell "Conjuring the Mystic Tower"?
To give some background, in our saga there is an old Covenant where the magi disappeared over a decade ago. They were rather secretive, and hadn't told anyone where their sanctums were, using teleportation magics to travel to and fro the farmstead that was the official address / visitor's quarters.
Circumstances are that we found the sanctums. In a small clearing in the nearby woods, is a large glacial boulder, and on top of that is a tower enchanted to be invisible. A couple of characters just happened to be looking in the right direction when some birds flew into the tower, which we confirmed after checking the shadows and counted all the bird skeletons on the ground.
As far as we can work out, the tower is made totally out of glass, in a single piece, with what may be chimneys for ventilation at the top, and a single brick melded into the base. The brick seems to be enchanted to make whatever structure it is part of to be invisible when looking in (except for the brick). One of the Magi was said to have a casting sigil involving glass and glassblowing.
No obvious doorway.
We have finally managed to get in, only to discover that the air is foul and unbreathable.
The SG says that the ventilation failed one night (we haven't found out why yet), and everyone inside suffocated in their sleep from the banked fires.
Then he told us to roll to see if the characters who breathed in the foul air have caught malaria.
In real life ventilation is very important to me. In a saga I think I can safely say it has never been of importance. But if it is a story hook it is quite reasonable for a SG to focus on it. However my own SG etiquette prohibits such a gimmick to ambush players: "So that wound you took to the arm last week was not specifically cleaned, now it gets gangrene." When nobody ever goes into that much detail with those matters.
The explanation of faulty ventilation killing people in their sleep sounds reasonable. But I think people venturing into a place like that should get a fair chance of senseing what is wrong before they are in mortal peril: Roll Per to notice, roll Stm to resist the first and mild effects, have a redshirt keel over to tip the magi off etc.
Ventilation normally never comes up because most people like nice windows in their mystic towers, and medieval windows tend to get drafty very quickly.
Ventilation does come up whenever people want to have underwater adventures or explore really deep underground. My saga's flying boat has a permanent chamber of spring breezes enchanted into it - I think we did it in case someone accidentally took the craft too high (we did get harassed by Infernal spirits of the air when trying to fly over mountain ranges instead of staying low to fly through passes), but our SG may have insisted on it for other reasons.
Keep in mind that most forms of construction didn't specifically need ventilation until the latter half of the 20th century, because gaps in doors, around windows, between boards, etc. allowed for plenty of ventilation. Most forms of stone are even semi-porous to gasses, and mortar doubly so. Of course if you go and make your tower from an exotic material like glass you aren't likely to notice those more subtle forms of ventilation (unlike chimneys which direct smoke away from a dwelling) are absent. So my answer would be that almost nobody would give a second thought to ventilation unless they have some specific reason to do so.
Creo magic is implied to be perfect, true; however, it will be limited by the casters vision of perfection, I think, because of how much control the caster gets on its design. While a general tower will have ventilation, a careless magus getting rid of drafty windows might make some errors based on bad rolls...
This feels like an excuse to roll finesse, but a lot of people don't like rolling finesse for Creo, understandingly.
With regard to the interaction between finesse and Creo, page 61 of HoH: societates has a text box labeled "Ease Factors for Creating and Crafting Objects" where we are informed that "A character using Creo magic need not roll Finesse unless he desires the finished product to be of a quality higher than that represented by an Ease Factor of 9 on the following table."
So, when they're needed, the finesse scores needed for Creo are the same as the finesse scores needed for Rego, and they're needed when a finesse roll of 9 or higher is necessary.