I always assumed that the inverted castle destroyed the normal one (and before knowing about it, thought that God or some divine power did it). Never thought too much about why.
Maybe because both are connected some crazy supernatural thing happens? Also I already thought that this light must be the same light that hits/comes out Dracula after you defeat him, so this can be symbolical because what Dracula says at the end.
I also think that they only thought it would be cool and less expensive to do it like this, since they use the same ending every time and it hides the inverted castle lol