According to Genesis, God told Noah to make a window for the Ark’s side. Genesis 6:16 includes an instruction to make a window, or opening, and to finish the Ark to a specified height above it.
English translations differ in how they render some details of the verse. The Hebrew term is often translated as “window,” although interpreters have discussed whether it refers to a source of light, an opening, or a roof-related feature. This translation issue is separate from the doorway mentioned elsewhere in the same verse.
The window later becomes important in the narrative. After the waters began to recede, Noah opened the Ark’s window and released the raven and then the dove. Thus, the opening functions both as an architectural feature and as part of the story’s observation sequence.
The answer is not the Ark’s door: Genesis explicitly treats the door and the window as different elements.