This is the place where Martin Luther, according to tradition, nailed his 95 Theses on the door in 1517. The original door was destroyed by fire in 1760 and has been replaced by a bronze memorial, built in 1858...inscribed with the theses in Latin. Luther is buried below the pulpit in the castle chapel.
As we know from church history, Luther's Theses questioned the Roman practices of that time, especially for the selling of indulgences to forgive sins, giving the buyer, with enough money, reduced time in purgatory for past sins. He also opposed the Roman law forbidding the clergy to marry. He rejected anything that obstructed a person's personal relationship to Christ.
Tradition is strong that Luther nailed these 95 theses on the door to the castle which was the place where public notices were posted. However, there are many scholars who refute this tradition, claiming that Luther would not have taken this approach to his protest. There is no absolute proof that he actually did nail these documents to the door. However, it makes for an interesting story.