[2023] Structure collapsed. Don't understand why.

It sounds like you literally undermined your tower. You can build infinitely high if and only if there is an uninterrupted column of blocks underneath each voxel, all the way down to bedrock. Otherwise, structural integrity rules are going to kick in, and those tall pillars of blocks are going to rely on the dirt/stone or whatever’s adjacent to hold them up against falling into the hole.

So you can have a super tower over an underground base, but you’ll want to pay close attention to its foundation, and not go mining under where there are tall pillars of blocks. It would be easiest to do any mining first, before you build up. And while there are no compression forces (you can build on a haystack), I personally would dig out the dirt/stone and lay a foundation of player placed blocks as strong or stronger than the tower walls. That would give peace of mind that errant zombie hits or pickaxe swings won’t be enough to destroy a block underground and trigger a collapse.

Oh, and don’t plant a rooftop garden if you’re still struggling with this stage of building.