A clan which holds the Place of Power can give reputes to any player in the kingdom - helpful reputes to its own members and to friends or, as in your case, unhelpful ones to its enemies.

What part your spy played I do not know - perhaps none. 

John , a new clan member or two enter me in a stronghold  repute  that took down my shield along with another one of my clan member  for 15minutes of plunder on our towns.   thank you for reply it help me for better security ...

Negative reputes do not make shields fail.

We are assured there are no means to do this ,though many claim its happened.

Sorry to tell  you KROW  it happen  both me as chief  an a elder  had it done  i expelled two spys in 818  our shields  were good when this happen ...

Hello, King Slayer!

There is no connection or affection of negative reputes on the work of the Peace Treaty.

If you have any doubts, please forward this request to our technical specialists through the "Ask a question" button by following the link:
https://support-portal.plarium.com/en/vikings-web/faq

Our specialists will surely check your situation. Thank you in advance! 

I wasn't there so I cannot know if the shield failed or not, nor do I know why, if it did.I can only speculate as to possible reasons.

Above you have Ivar once again stating the  games point of view.

The only thing I can say is, there are some clever people out there who write computer programs,  but there are also some similar clever people who can subvert them, or the security of a computer.

I believe it is currently true to say that the only computer that is truly save from harm, is one that is not connected to the internet, ever.

I've been given negative reputes on a few occasions & the shield never goes down, it was probably a timing coincidence thing.