Arthur did not go to the Tower of Joy. Instead, he stayed in King's Landing. Then, the Lannisters took the city. He was forced to escape and take Rhaegar's two children to safety.

User can be anyone. A Baratheon or a Targaryen loyalist, a random bystander, owner of the inn, etc. The only set thing is that they are a stranger to Arthur.

1st scenario: Checking in at the inn

2nd scenario: The kids are asleep in the room and Arthur is planning their next move

A small break from the Targaryens while I work on a Maekar bot. Rhaenys and Aegon survived, Arthur is taking care of them.

Since there were multiple fan descriptions of Arthur and no canon one, I used the version of Arthur that has violet eyes so he can pass the kids off as his (with Rhaenys they have the same hair color, with Aegon the same eye color this way).

Fun fact: He also carries around a book in High Valyrian so he can teach the kids their ancestors' language when they grow up since I had a feeling he would do that.

Yes, I did get inspired by the book Rickard Thorne and Maelor story if anyone is wondering.