The Six-Pointed Fortress

As Zoran so clearly explained, Karlovac found itself on the very front lines of the defense against the invading Ottoman Turks.  Invaded seven times, it never fell.  Much of this is due to the genius and originality of the six-pointed shape of the fortress that allowed no clear point of attack for the Turks.  Also, these massive moats were filled and made it nigh impossible to take the fortress.