Can't build, but also can't find red block?

I just spent a long time building this castle, but there’s an error somewhere and I can’t find it. Anyone have some spare time and want to help me out? I’m beating my head against the wall. (9.6 MB)

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Turns out it was the stairs. For some reason the auto-stair-builder built the steps extra deep and built them right into the floor of the level below, which meant there were multiple blocks in the same place.

It never did show up on my original, but I saved it as a template and built it somewhere else, and the red showed up there.

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This seems to have been caused by trying to build stairs approximately over the hole I’d made for the set of stairs below that one. (Multi-level building.) I’m assuming the stairs “saw” the hole and miscalculated how long they needed to be.