When Prophet Ebrahim (عليه السلام) built the Ka'bah there were originally two doors. One is where the current door exists, the other on the opposite wall.
This exposed view of the Ka'bah is the corner of the Rukan Yamani. You can clearly see where the second door used to be located to its left.
The door was first sealed by the Quraysh when they rebuilt the Ka'bah during the lifetime of the Prophet (ﷺ).