[RMXP] SIDE SCROLLER??

author=meaka
There may be an easier way to do it, but what I've found works for me is setting a blank area in the tileset as impassible, then in either layer 2 or 3 of the map editor, laying that across the map so the character will move left and right but won't walk up and down.

Essentially, you wanna set your character on a rail! This is a quick visual representation with the impassible "bumper" tiles being the red lines. You can get fancier with it if need be.

author=LockeZ
Aside from adding invisible walls to create a horizontal path, you probably also want a custom movement handler so that pressing the down arrow ducks and releasing it stands back up, and pressing the up arrow doesn't change which way the player's facing. This can be done with some simple edits to the scripting starting around line 255 in Scene_Map.

You can probably also do it without scripting, just using common events, but based on your original question it's pretty clear you aren't comfortable using events yet either. If you have to figure something new out either way, it's probably more useful to figure out the scripting in this case, since you'll get a cleaner result.

If you add a Pixel Movement script to your game, which you probably should for this type of game, you might have to make changes to that instead of to the built-in code in Scene_Map.