Probably most of European citizen live in 6-story high cities, aka Paris, Barcelona, Lausanne… and 6-story is the optimal density for having a lot of choice of

Probably most of European citizen live in 6-story high cities, aka Paris, Barcelona, Lausanne… and 6-story is the optimal density for having a lot of choice of officrs (second floors), a lot of commerce (ground floors), and a lot of housing (the rest). With that, you walk or bike to work while still working for a high pay.