Define a no-go zone with proper criteria, then bring up clear, properly identified examples within Europe. Otherwise you're arguing for an abstract without proof that doesn't need proof either.
an area, where people of a certain ethnic, color, religion, sex, sexual orientation, political opinion, beliefs or just foreigners and non local persons have a (way) higher chance of getting into conflicts and / or being harmed
the usual stuff like extremely high criminal rates.
no obligations to the law and law enforcement, rather resistance and attacks
places where everyone likes to spend his holidays because it's so save and friendly there...
i'm from germany
we have criminal clans "ruling" certain areas in marxloh or neuköln
rocker clubs and their bars / streets in bottrop and other cities
neonazi hot spots in barop and certainly also the east
and now imagine great places in russia, serbia or istanbul for example
but sure everyone in europe can go everywhere in europe and doing everything they want without being worried.