"Prison" (1949) is the first film in which Ingmar Bergman develops something like a style of his own.
We see various elements that reappear in later movies. There is the character of death (to reappear in "The seventh seal", 1957) and dreamsequences (to reappear in "Wild strawberries", 1957).
The most important sign for things to come is however the theme of the film. The film is about a director trying to make a film about a world governed by the devil. Is a world governed by the devil not very much like a wordl where God is silent? Later Bergman would make a trilogy around this theme consisting of the films "Through a glass darkly" (1961), "Winterlight" (1963) and "The silence" (1963).
So in "Prison" Bergman started to experiment with a personal style, but this style was not fully developed yet. There are good sequences (such as the dream sequence) but as a whole the film is somewhat cluttered and inaccessible. Moreover the film in a film format about a director struggling to make a film does not work very well in "Prison". It does work very well in "8,5" (1963, Federico Fellini), but the Bergman of 1949 is not the Fellini of 1963.