Won't they stop building the floors a few tiles out from the windows?
Yeah, but a tower around the outside tower then using pump projection of liquids to cause obsidian falls over the area where the invisible bridges are, should neatly deconstruct/melt them. If not, an obsidian tower from bottom z-level to the top z-level could easily be smelted and the roof accessed anyway.
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It gets harder with no magma. You need to bridge-a-pult military dwarves and construction supplies across the 10-tile gap that the invis-a-bridge generates on the top level. I'm estimating kill numbers at... say 50? at a conservative estimate. Probably nearer 100.
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Could someone look at what can be built at the top z-level? In particular, things that block dwarves and arrows:
I know walls can't... but:
Windows (IIRC no, but maybe...)
Raising Bridges (and retracting ones too for that matter)
Statues?
"natural" obsidian walls?
If raising bridges can be created at the top z-level, then I guess I'll have to concede and say it's possible to make an impregnable tower if the map has no infinite magma source.
With magma, I see no way a prospective tower-cracker can't simply encase the entire tower in obsidian then walk across to the top level and drill down.