Phot,
At this time there is no single set of rules for General Aviation in Europe; each State (country) will have its own regulations; you will have to be more specific if you want information.
The word 'city' also has no meaning in aviation regulations; for Europe, and specifically for Commercial Air Transport, the issues are: the Rules of the Air (which once again are State specific); and, the ability to perform a safe-force-landing following a power unit failure. There are many cities which have the equivalent of heli-lanes which provide a safe route through a city for singles - for commercial and private flying.
Jim