Thanks guys.
Right now, the central "tower" is the primary area for the magi (library, council room, etc.), though lab and sanctums will have to be in one of the large wings just for the amount of space required.
As for the "tower", there is a lot of room for play in its design. With little to no modification to the drawing, it can either be considered a large, single-story domed chamber, or it can be considered two-story. If two-story, we could add a balcony over the external columns, as well.
In my early design, I was actually elevating the windowed-level of the tower so it rose above the roofs of the other wings, providing the wizards a 360-degree view of their surroundings, but to do that would require a 40-50 foot structure (taking roof and foundation into account), and that would be difficult to conceal.
However, there is one caveat there... although the covenant may remain "secret" early on, I firmly doubt that, without magical concealment, we can stay on that cliff, right next to a relatively well-travelled road, and remain unknown for the long term. If we, instead, aren't concerned specifically with remaining hidden, then height constraints need not be too grave a concern. As this is definitely not a fortified structure, we shouldn't need fear a military reprisal. And once the labs and sanctum are secured underground, we may very well be able to pull off the surface structure as simply a fancy villa for affluent scholars with a love of all things Roman.
FYI: For magical enhancements for defense, I was considering having an underground room that brushes close along the bottom edge of the cliff, where the road passes, and have a magical Muto of the earth to allow anyone in the room to "see through" the earth, thus being able to watch the road without being visible themselves. Comments?