[BeniBoy]

The ground. Paris's ground is not strong enough to support high rise building especially in the southern part, where a lot of it have been excavated in the last 500 years (and filled with dead bodies, ie the catacombs). Of course with clever engineering everything is possible, but that really limits the development of such towers.

It is true that the main limitation still is the construction code, which preserves the global proportion of the city.

[Scoundreller]

The usual approach is to punch pilings into bedrock. Nothing new really. Hollow shafts might be an issue, but nothing a stroke of the pen won't solve.

[lazyasciiart]

Gets a lot more complicated when you have tunnels everywhere already.

[Scoundreller]

That’s what I meant by shafts. You could fill them, but dunno how kosher that is. It’s not like they’re open to the public or anything.

I believe you can take the catacombs under Montparnasse station, so it’s a solved problem ( I hope ).

[lazyasciiart]

I'm not at all sure what you mean. There is no way you could fill the catacombs, if that's what you're suggesting. And how does having catacombs under montparnasse show that you can put pilings through train tunnels?