This is a narrow and difficult site to work on, as explained in this article (in Fr, don't think about it if you can't read the language).
https://lagazette-ladefense.fr/2019/...s-du-quartier/
There'll be only 2 tower cranes to build it, so it won't be super fast.
The footprint of the project itself is narrow and the work to refurbish the Pascal tower nearby doesn't help them, but the worst would be by far constraints due to infrastructures on site.
Indeed the underground is like Swiss cheese on that spot, with various tunnels crossing through it from side to side, including some meant for transit/trains, thus requiring maximum safety.
And there's some work to redevelop the surrounding ground too. They want some kind of pedestrian promenade instead of what was once an expressway of sorts.
The result is a logistical challenge giving engineers headaches. They say they have to update scheduling every hour or even every 30 minutes.
In a nutshell, that will be a hard site to work on when hundreds of workers get there to erect the tower.
From the project Twitter account.
https://twitter.com/HeklaTour/status...62052100096000
The site is looking neat anyway, just as usual over here.