Hey guys, I was confused by this
battle report from SN Studios. I've linked to a timestamp of the principle example. Basically, they were both acting like monstrous creatures couldn't enter the ruin. I wasn't sure if the model literally wouldn't fit (though it looks like it can if you rotate it), but
then they pointed out this example where the Keeper of Secrets couldn't charge into a ruin that was basically a crater with some chest-high walls. The game literally hinges on this rule, and the Slaanesh player was hot off a tournament win so he should know his stuff.
My understanding was ruins are area terrain, and the area terrain rules say "Models move over Area Terrain using normal rules for movement." Those normal rules for movement say "Models can move freely over terrain features 1" or less in height."
You could maybe make the argument that the height of the walls applies to the entire piece of terrain because Dense and Obscuring keywords say "The height of a terrain feature is measured from the highest point on that terrain feature." However, the Scalable rule says "Only INFANTRY [...etc] can be set up or end a move on the upper floors of an Area Terrain... (other models can be set up or end a move on the ground floor)." It calls out that any model can access the lower levels.
So... what am I missing?