A mage invents a version of CTMT, do all instances of him casting the spell create identical towers or could he vary the design? If so how much?
Could he create a tower with only half the height with the same spell or would it still have to be about the same height. Could he vary the architectural style between castings? Could he vary the type of stone used such as granite for one tower, marble for another, sandstone for a third, etc?
If only a tower, how much can he vary the internal walls, the floor/ceiling heights and so on?
The spell includes three magnitudes for "elaborate design" so I think you have almost complete control of what you do within the maximum volume. If you want to change the kind of stone from casting to casting, I would add another mag.
It gets fuzzy because I have never seen a canon creo spell that made multiple different things, at the same time. Seems, to me, to be an "unspoken" guideline. I would love to see a ruling the other way. CreoTerram all the gemstones you'll ever need!
I'll have to check my sources, but I read somewhere that Ancient Greeks postulated that minerals grew, much like plants, animals and Man. And that if you let a mine heal, it could grow back. I thought it was neat so ran with it.
So in a one off I ran, i added a Mystery that allowed the use of Creo Terram to "heal" stone, minerals and the "heart" of a mine. Basically allowing the growth of minerals from existing minerals in their 'natural habitat', instead of spontaneously creating them from nothing.
Every time i try my hand at one of these sort of answers, everyone says I'm wrong but I'm going to take a stab at it anyways. If you learn the spell to create a tower, it is always the same tower. If you want to create a tower of a different size, that would effect the target of the spell which would be a different spell then the one you already know.
The mystic tower states that the mage decides the layout of the rooms and other inner furnishings of the tower. The tower might look the same, but I think this particular spell allows you a certain degree of flexibility in this kind of stuff.
I can easily imagine a way less elaborate level 25 mystic tower with a much simpler design and a totally unflexible interior design.
When using a ReTe spell you do not need a specific spell for a pebble and another one for a boulder as long as the magnitude covers the amount to be moved. When creating a hole by PeTe you can influence the hole's dimensions to the maximum given by the guidelines used to create the spell. When creating a wall by CrTe you can adjust it to the trrain and decide things like in which direction the battlement faces. (See HoH: TL, the Tremere Chapter. Serf's parma, I do not know the pages...)
Having said this, I would allow my players to cast their spell and use a finesse roll to design the interior and outlook of the tower. And yes, this roll can be botched... (and was in our saga...)
Whether or not they could change the material the tower is made of, is more difficult and depends on the saga. However, as long as you stick to "hard stone" "soft stone" etc. this might not dirupt your saga.