Malbork Castle

When, after the Battle of Grunwald, King Władysław Jagiełło and his Polish-Lithuanian troops besieged Malbork Castle, there was a traitor inside the castle who hung a red flag in the refectory window.

An excerpt from the article 24 facts about Malbork Castle

It was supposed to be a sign to the besiegers that the most eminent personalities of the order were gathered in the summer refectory. The besiegers fired an 80-kilogram bullet that was supposed to hit one of the pillars supporting the entire structure. However, the bullet slightly missed the pillar and hit the wall above the refectory fireplace. To this day, a part of it is stuck in the wall.