Welcome to HBGames, a leading amateur game development forum and Discord server. All are welcome, and amongst our ranks you will find experts in their field from all aspects of video game design and development.
I can't for the life of me find the side wall tile in the Castle In tileset.
(Meaning, when you're doing the walls, the walls that go on the left and right.)
There's no such thing. The only walls that you see in RMXP, based on how the tilesets are constructed, is the front wall (when you're ouside), and the back walls of rooms.
You do realise that it auto-corrects itself, right? You just have to fill the entire map with that autotile, and then put whatever floors and walls you want on the same layer...