conclusion: those aren’t mountains. just really, really, really big rocklords

@Hateot  
Ask the animators.

@Jarkes
 
Though this wouldn’t explain why the first shot doesn’t have the mountain with the waterfall running through it…

Now this is just damned confusing.
 
Season 4 and 5 castles have a lake behind them.  
Rainbow Rocks castle has mountains behind, but the Season 5 castle has only the one with a waterfall on it.

@Kolbjorn  
What about the Figaro Brothers?

It looks like Howl isn’t the only one with a moving castle

You know, it could just be that the second is a shot from a further distance, and the third is a shot from a closer distance…

@SeraphimDawn  
Bigger Than Your House.

It’s the castle that’s growing like crystal.

@Background Pony #8797  
I think this is more of an issue with the graphics department.

@Background Pony #8797
 
That would be animators in this case.

Even better question: How big is the castle exactly?

Soon the mountains will consume us all

constantly proving the writers don’t care anymore

That’s weird.
 
At least it’s always next to a lake.

Not even trying…