One of the obvious lessons to be learned from the almost comical ease with which Prince William's 21st birthday bash was infiltrated by a "comedy terrorist" is that the royal palaces and residences have to be made more secure.
But another lesson to be re-learned is that royal security can only go so far.
The royal family depends on being seen. It has to get out and about.
It is there, on the streets, that it is most vulnerable. And yet, it is there that it has to go.
The Queen is determined to keep on riding in open carriages during state events.
If one of her guests objected, then perhaps the old glass backed Rolls Royce would be wheeled out. It would not be quite the same.
The Queen insisted on an open carriage for Ascot last week
So amid the witch-hunt atmosphere following the nabbing of Aaron Barschak, it is worth remembering that a real terrorist would not have to dress up like Osama bin Laden, prance into a palace and blag his way past a na´ve policeman as Mr Barschak apparently did.
I have been all over the world following members of the royal family. The whole point of their visits is that people should be able to see them.
The royal family has little other reason for its existence than the link it can establish between itself and the public.
If the link is confined or broken, then such visits would be rather pointless.
There would certainly be very little television coverage if royal encounters were kept to tedious receptions.
Even Queen Victoria, who had real political power unlike Queen Elizabeth II, realised that. When she withdrew from public view after the death of Prince Albert, she became unpopular.
Certainly, there are royal events where it is impossible, or should be, to get in without invitation and checks. Prince William's 21st was one of these.
Getting in uninvited to a state dinner in the centre of the Sri Lankan capital Colombo on a visit by Prince Charles in 1977 would have been quite a feat.
Whole blocks had been cordoned off in case of attack by the Tamil Tigers.
But go on most royal visits and there are walkabouts into crowds which have not been checked.
Policemen discreetly watch the people but discretion is the whole point. That is how the Queen likes it.
In contrast, security is the whole point when an American President comes to town.
When President Bush gives a speech in public, every member of the public is searched. That is how the US Secret Service likes it.
As for the royal residences, it is clear that some tightening up is needed.
But the low security philosophy set by the Queen herself tends to percolate down to the officials on the door.
They don't want to be too rude to the Queen's guests and that is probably why Mr Barschak got through.