One of the writers of the film, Igor Ugolnikov said that making a film about WW2 is difficult, as it shows "how lousy a person you are", meaning that you must not let *that* tragedy and *that* heroism appear as farce, exaggeration, and add stuff to the story that would make it more appealing or sexy to viewers. This attitude does show in the film, and the authors did they best to adhere to it.
The Brest Fortress is a symbol of what defeated Hitler's armies in Russia. It is something that many people nowadays refuse to acknowledge. It was not cold winters, poor roads, long supply lines, or Hitler's bad decisions. That was human spirit - of "subhumans", "Bolshevik scum", and "Eastern hordes". And this is what the film depicted best of all.
On the Russian website dedicated to the film, people discussed all the details a lot. The film does have inaccuracies, but they tiny. For example, Germans were not known to use human shields near the Brest Fortress. Komissar Fomin was executed by Germans after being betrayed by one of his subordinates, not after declaring himself to be a Komissar and a Jew.
All the main events shown in the film are all real: there was panic originally in the fortress, but it was subdued eventually. The fortress held out for over a month - there was still resistance going on well into August (not 9 days as the summary above says!), long after most of Belarussia was captured. The names and stories of all the main characters are real: Lt. Kizhevatov was killed 7 days after the invasion, his mother, wife, two children surrendered and left the Fortress (but were executed later on), Komissar Fomin (he *was* a Komissar) was executed by Germans, Major Gavrilov, severely wounded, was captured by Germans in late July and was held as a PoW until the end of the war. Failing to capture the fortress, Germans dropped two-ton bombs on it, but the defence still went on.