The latter is easy enough to exaggerate - it's not just "marble", but highly polished, truly impressive marble, etc etc. And perhaps easily changed in color to all the varieties marble comes in. Such an artwork would be highly prized among some magi, and impress the pants off visitors.
But perhaps that's not "major", or not by itself. And yet, you don't want to give too much away - but that depends on your saga.
+1 Living Conditions is fairly important, especially to old(er) magi.
+1 general bonus to labs is big too, and something many magi willingly spend a couple seasons to achieve. This would be from the mystic shape and balance of the rooms, etc.
You could toss in a "special lab bonus" on any activity involving Rego or Terram - studying, studying from Vis, writing about those Arts, teaching those Arts, and any and all lab activity involving them - the size of the bonus is up to you.
You could combine several of those, until you feel something fair and "major" is achieved.
Or, you could go a different way - perhaps the caster has an innate "connection" to the Tower, a sort of Intellego aspect to it, and, if touching the tower, can take an action to "concentrate" to sense who is in the tower and where. Details up to SG (things like this can often be better left imperfectly defined - it comes and goes).
Perhaps with respect to the Rego part of the mage, the caster can, with concentration and slowly, over an hour or a day, alter the tower's configuration. This could create sealed chambers, modify rooms and floors, combine or split rooms, etc., as needs change over time. (Perhaps scale the speed to the size of the change, so a cubbyhole or a small window could be opened and sealed fairly rapidly, but smoothing the main staircase into ramp would be a bit slower, and changing the main stairs from clockwise to counter-clockwise might take all day, etc.)
(A more extreme version would allow the tower itself to move over time - not sure I'd want to suggest a "walking" tower, but one that can move slowly, a (few) hundred paces overnight, just slow enough that it can't actually be seen as it happens - that could be fun.)
Or, with regard to the Terram part, perhaps she could mix and match the material of the tower to stone/glass/metal (within reason?)?