In Genesis, the Ark’s stated entrance was a door. Genesis 6:16 instructs Noah to make a door in the side of the Ark, rather than describing multiple entrances or a named gate.
The same construction passage describes the Ark as having lower, second, and third decks. It also mentions an opening or window near the top, which served a different purpose from the side door. These features are sometimes combined inaccurately in illustrated retellings.
When the Flood begins, Genesis 7:16 says that those entering the Ark went in as commanded, and that the Lord shut Noah in. The wording gives the door a narrative role as well as a structural one.
The account does not provide a modern engineering blueprint, and the Hebrew terms used for some features can be translated in more than one way. The side door, however, is the clear conventional answer to the question of the Ark’s entrance.