In the scene at the Sylvester evening the camera shows the sky and it's snowing, but there are just a few clouds in the sky so it can't really snow.
Saxony didn't separate itself from the Holy Roman Empire. It was separated after the Rheinbund was founded in 1806, to which Saxony was included later. Also, in the movie the inclusion of Saxony is mentioned by King Franz II. In real life it didn't happen until December, almost four months after Franz's resignation.
When Napoleon is visiting the grave of Friedrich the Great in Potsdam he says, "Would I stand here if you were still alive?".Napoleon really did visit Friedrich's grave, on Oct. 25, 1806, and actually did say words similar to those ("You wouldn't have come here if Friedrich would have been alive"). However, he said them in Friedrich's flat in the Potsdam palace, not at his grave.
August von Gneisenau was 47 when he became commandant of Kolberg, not 32 as stated in the movie.
The words of the resignation speech by King Franz II in the film are different than the ones in the actual speech.
Nettelbeck was arrested by Lucadou after he threatened to kill him with his sword for saying that they might have to "eat crow". However, in real life this happened not after a disagreement about transporting the cannons to Kolberg but after after the argument about Lucadou's command to rip off the pavement. Lucadou said the line in real life because during that moment a bomb exploded nearby.
During the summer party in Kolberg Nettelbeck said that Prince Louis Ferdinand of Prussia died. However, Louis died in October of 1806, not during the summer.
Some of the lines from Gneisanau's motivation speech in Kolberg in 1807 were actually from the appeal "An mein Volk" (To my people) which was written in 1813 for King Frederik Wilhelm III.
In the first scene set in 1813 Gneisenau mentions the compulsory military service that existed at the time. Even through the first mention of the military service in Prussia was in 1813--provided by the Reforms of Prussia--the actual start of it by law was in 1814.