How about an option that will allow troops to flee when an attacking force is approaching a location you are harvesting.

To make it fair and more challenging perhaps have only 50% of the resources harvested taken to your castle and the attacking group will get 10% of what you harvested to take home automatically only if it is during a KvK, KvK Retribution, or OvO type event.

Sounds like a win/win.

You can make this a purchaseable option through personal points, your store, the in-game item store for gold, and gained through treasure chests of assailants or special contests.

I suggest making enough of them available to make it interesting so that the attackers will not know what to expect.  I would expect the end result would be more attacking of resource locations during events because they can always get something out of it.