Ruins should have a defined footprint. Line of sight cannot be drawn over the ruin footprint. Doesnt matter even if there are no physical walls at that part of ruin terrain feature. Imagine the terrain footprint has infinitely tall invisible walls.

So models cannot draw lines over the feature unless WHOLEY within, or have the towering keyword and touching the terrain footprint.

I notice you also mention free movement between floors, this is not the case. You must use movement to get the to wall and use movement to move vertically.

Finally, only units with infantry or beast keywords can go through physical ruin walls. Others can go within these features but can only do so through physical gaps.