In Secrets of Dumbledore, we see that Yusuf Kama was able to enter Nurmengard Castle's exterior courtyard without having to bypass any protective enchantments like the Death Eaters do to Hogwarts before the Battle of Hogwarts in Deathly Hallows.
Why would someone as intelligent and powerful as Gellert Grindelwald not protect the castle from unwanted intruders? Did he not think that by leaving the castle unprotected, that he could be killed whilst asleep?
