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I'm saying it again: you place an object in the Dungeon, for example, a column. For it, select "Shared" and scroll down to choose the object "downbridge" (there are several of them). It will help!
Added 1 minute ago
Here's how it works. I spent a long time looking for how to do this. It seems like you can unlock them somehow.
Here's a small mod; copy it to the Editor folder, replacing the existing files. Version 3.0 or higher is required. It unlocks all objects that can be found in "Shared" (or almost all of them).