Thanks CA.

You got me curious, and I just reimported that tower model into Simplify. It displays weirdly, as if it has all kind of flipped normals, although it doesn't. It displays as if a number of faces are missing - yet Simplify had no problem slicing/printing it, it just wouldn't let me add supports.

I just tried the Repair>Flip Surface Orientation command, and that made the model display properly, and then I could manually add supports.

Bit odd - if Simplify thought the model had flipped normals, I'm surprised it would print at all. But if it happens again, at least I'll know what to do.

Thanks again!