Thinking back on it, If I was to do it all over a third time, I think the way to avoid hand measuring each and every crenel while still getting the organically-curved wall would be to draw the whole wall down a straight line first, then add slices, and bend it on those slices.

@jean lemire said:

See this SU file for ideas.

Neat. That's as close to what I was hoping to get originally. Wish I'd seen this last night. I ended up rebuilding the thing from scratch after all.

Here's what it looked like after the rebuild:

That one also shows better than the original image that the walls also needed rebuilding for slope, not just the towers.

I ended up taking the base of the castle from the original sketch, to keep that organic shape, then rebuilding the follow-me shape to drag around that, and applying it anew. As before, the biggest pain was putting the crenelations back in. With the way the wall is curved, a simple array of lines won't work as crenelation bounds to push/pull on, since the array won't follow the bends in the wall. If there's a way other than going around the wall and measuring out each line by hand, I don't know it.

Even so, and even with my newness to sketch-up, it all took under two hours. That's good. And it was galling me that the answer to such problems in sketch-up was "just rebuild it from scratch!". With your answer, it seems you've shown a real way to have built onto what I already had, rather than throwing it away and starting over. (Specifically the second version.)

@unknownuser said:

Can you share the troublesome portion?

The bend in the towers and the walls. There's a picture and longer description in the original post. As for other ways of making the towers, there are many, but the intent is to fix the existing graphic with a few clicks by changing the FollowMe profile, not to rebuild the model from scratch.

I have a shape, somewhat complex, that was built by dragging the follow-me tool around a slightly-less-complex shape. I've built onto the model since then. And now I need to go back and change the profile of that follow-me, back at the dawn of time.

This is what the model looks like presently:

I've sent it off to get made into little plastic versions for a board game I'm making. But the printer has written back to say that their plastics manufacturer is going to have to charge me a lot more money to make the mold, so long as those arcs are in the walls and towers.

OK, no arcs. They can be straight lines, so long as the walls are thicker at the bottom than at the top.
(See the arcs? The walls bend out to be thicker at the bottom than the top. The goal is to kill the bend. Make is a constantly sloped line from top to bottom. Not 90% perpendicular to the ground, but gently sloped off the perpendicular.)

Thing is, I don't know how to take the arc out of the follow-me profile now that it's done. But surely there's a way to do it. There is a way to do it... isn't there?