Ivy I would probably count the same way as clothes do when you cast an invisibility spell on person/s using Individual or Group. Technically not inside the tower but since the invisibility is going to include the tower itself and not just the things inside it makes more sense if it behaves closer Individual than certain other uses of the Structure target.
EDIT: I definitely would not stop a person from inventing a spell that specifically did not make the ivy invisible as well if they thought it was cooler to make it so the ivy looked like it was growing in air but that’s a particular case of a player wanting to do something fun by a particular sense of fun.
I suspect it is not quite the same case.
Take this clothed invisible person, standing next to some growing ivy, then ReHe the Ivy to wrap around and attach to the invisible person.
Should the ivy be invisible? (at the next casting of Invisibility)
So the constant effect refreshes twice a day. How fast does ivy grow in half a day and on that first day when the ivy has started growing on the building it will be visible but that sunrise or sunset it is now like clothes worn at the time of casting.
The stranger issue seems to me to be the windows where light will stream out at night, not being shaded by the invisible structure.
There is a ring/circle spell, ReIm I think, which prevents the species from escaping the circle. Thus the inhabitants can see each other, while remaining visible to the outsiders.
I think a structure variant of this would be possible.
To me, the contents of the structure is part of the structure, so neither light nor people can be seen through the windows. However, living inside an invisible structure must be quite a taxing experience.
The light won’t be invisible though, Im doesn’t affect light, that is clear by the rules so windows that aren’t solidly shuttered will have light spilling out of them which is probably not an issue during the day but at night it would be.
EDIT: this being sort of the opposite of an invisible person still having a shadow, the building provides a shadow from these indoor lights everywhere but the windows.
RIght, so this would result in a eerily lit area around the tower, with no discernible light source I suppose. If Im does not affect light, then light is not species, and hence cannot be seen either. Thus the light source would be invisible, but the light causes the surrounding land to emit species.