There are numerous references to doors in "The Metamorphosis," but the three doors that surround Gregor Samsa's bedroom are particularly pertinent. Indeed, they metaphorically make clear that Gregor is a prisoner of his family. Three of the four walls that make up his room have doors; there is one double door that leads into the sitting room and two side doors. The fourth wall has a window facing a hospital, but it never occurs to the Samsas to take their son there.
The first person to knock on one of Gregor's doors is his sister Grete who chides him gently for not getting out of bed. She needs him to get up so she can continue to have an easy life, with her violin lessons paid for. At the beginning she seems to care about Gregor by feeding and cleaning up after him, but when it seems her brother cannot ever again be useful to her and the family, she disregards him. The mother is next to knock on the door, pleading with Gregor to get up and go to work and covering up for him when the Chief Clerk comes knocking at yet another door. The father stands outside the third door ordering his son to get up. Gregor, however, has taken to locking all the doors at night. And although this might seem like a powerful act, it simply means that Gregor lives in a locked box, with his family outside waiting for him to go to work and support them.
Gregor Samsa lives like a prisoner in his bedroom imprisoned by his own family. He is trapped not just physically but also emotionally. There never has been anything stopping Gregor from escaping the life forced upon him by his family. Indeed, he travels by train all the time. Why not just sit still and escape to whereever the train goes and start a new life? But for Gregor Samsa escape proves impossible because he doesn't seem to realize he is imprisoned. He loves his family and does everything in his power to provide a pleasant life for them, sacrificing himself in this effort. After his transformation, he thinks only of them and panics when he thinks of how they will support themselves. Even when Gregor finds out that his father withheld money that could have been used to alleviate Gregor's burdens, he cannot feel anger. How then, can Gregor escape the horror that has become his life? When Gregor transmutes into a gigantic insect, he does manage to escape his job. Now his family has to support themselves. However, how then is Gregor going to escape the family that fails to care for him in turn? The only real way for Gregor to escape is in death.
The Metamorphosis: Metaphor Analysis