Plot hole: The huge guns are set high up on a cliff face facing out to sea and it is obvious that they cannot be depressed to fire at a downward angle - the massive gun carriages set on rails would prevent that happening. They cannot be elevated to fire at an upward angle, too, because they fit pretty snugly in the hole cut into the cliff face to accommodate them. This means that their maximum and minimum ranges would be quite close together, covering a strip of maybe a few hundred metres either side. Given that the sea is completely open on the side of the island they are protecting, why don't the ships targeted by the guns while passing the island simply sail inside or outside of the narrow stretch of sea the guns can hit?
Plot hole: Maria describes in lurid detail how Anna was arrested by the Germans and tortured by being severely whipped, leaving her with gruesome scars all over her back. She also says that Anna hasn't spoken a word to anyone since she escaped from captivity. How, then, does Maria (or anyone else) know about the scars? Nobody saw them (they don't exist) and Anna obviously didn't tell anyone about them.