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The only reason anyone could call Hogwarts 'the safest place in Magical Britain' was because of its staff-members. Never underestimate a master of a craft, who's then spent years teaching others in that craft and slowly accumulated even more creatively destructive ideas from their students.

But as long as you didn't arrive at Hogwarts with Dark magic, or with malicious intent towards its students or staff, the wards didn't really care more about you than they did about mosquitoes.

Certainly, someone could've tried turning Hogwarts into a fortress, and it did have some good defenses in the case of an outright war breaking out, but Hogwarts was first and foremost a school. And schools needed to be able to accept new students, and guests, and new staff-members, and-...

It was possible to simply screen everyone entering and leaving the castle's wards, but with how many people that amounted to, it wasn't exactly a painless procedure. And it would probably need to be updated regularly, weighing in between making sure that it allowed students to return from Hogsmeade weekends, and that it wasn't completely ineffectual at stopping graduated alumni – which would be most every wizard and witch in the country.

The basics of it translated into the idea of trying to frisk anyone stepping onto King's Crossing. Sure, it'd make everyone 'safe', but it'd cost a hell of a lot of time and effort, be hilariously inefficient, be a massive inconvenience for literally everyone involved to the point of violence, be highly likely to be abused for the sake of molestation or blackmail, and maybe stop a 'danger' once every decade-or-so since the people actually introducing the dangers would know to think of a way to bypass being frisked.

In short, Hogwarts had a lot of holes in its wards, because it was a school. And nobody was crazy enough to think that making it into an authoritarian fortress would be a good idea.

http://www.fanfiction.net/s/13318951/2/