@vonny,
Yep. I used to do some roof top gardens and substantial trees like those need proper support engineered by the building engineers. I'd be interested in how they rigged that. It's not just the trees, but the amount of soil necessary and it's weight when wet.
I don't think I saw that tower while walking Lucca and I walked it for four days, lots to see. I think I knew they were Holm oaks from years before and didn't connect them in memory when I actually made it to the city. So, I found the link by looking up holm oaks in tower.