Visiting the Two Towers in Bologna
Stericksburg actually has this same architectural feature, but I haven't discussed it much here. The wealthy prefer towers with second-floor entrances accessed by ladders, external stairs, and/or magic. This is originally for defensive reasons, but it also has an aesthetic element.
As such, the tower of Black Jans that comes and goes isn't that strange in that it's a tower in a faux-medieval city full of houses. It's that it comes and goes, is accessed by streets that aren't there when it's not there, and that first floor entrance that it has. That's a statement of power, right there - yes, I have a front door. Sure, you can come in. No, I'm not concerned you might cause trouble.
This clearly says that Black Jans is powerful. Of course, it might also say that the wizards of Cashamash aren't worried about peasant rebellions or hostile townsmen rioting so much as being able to get into any perhaps out of their towers quickly. Cashamash is a weird place.
The Felltower - city-with-towers connection wasn't really deliberate. I had that idea in my head since I'd encounterd it in history books and Glen Cook's Instrumentalities of the Night series. And then I went and
Observers might notice there were no towers in the Cold Fens, which is true. It didn't seem to come up there although the PCs didn't really explore the flooded swamp or the distant ruins within it very much. There were towers in the Lost City of D'Abo, though, which connected quite directly with themes and elements of Felltower . . . no need for Hidden Lore (Towers) or anything, but it's worth keeping an eye on such structures as they may not be purely coincidental choices of design. So while this is largely just an element of color for the game world, it's consistent with the non-color encounter areas that the PCs have interacted with.