Izborsk

In the later 16th century, Izborsk was one of the smaller, but nonetheless strategically important fortresses that protected the northwest Russian borders from invasion. The fortress was supposed to be impregnable, which is why the seizure of it in 1569 by a small Lithuanian regiment came as such a shock to the then ruler, Ivan the Terrible. The relative ease and suspicious circumstances of the seizure of the fortress deeply troubled the already paranoid Ivan. In the dead of night Teterin, a Russian turncoat disguised as an oprichnik, ordered the gates of the town be opened in the name of the oprichnina, thus allowing the enemy regiment to enter and overtake the fortress.

• http://en.wikipedia.org/wiki/Izborsk