According to Genesis, the Ark’s main entrance was a door.
Genesis 6:16 lists a door in the side of the Ark as part of its construction. The description appears alongside the Ark’s dimensions, decks, and window, making the door one of the few explicitly named structural features in the account.
Genesis 7:16 later says that Noah, his family, and the animals entered the Ark, and that the Lord shut the door behind them. This detail gives the entrance narrative importance beyond ordinary architecture.
The answer is not “gate” or “hatch,” terms that may appear in imaginative depictions but are not the standard wording of the Genesis passage. The text specifies a door in the side, while leaving many construction details undescribed.