@tom-barrett I agree with @Nxndraw about the monster "falling" off the page.
It is really hard to show a child imagination in one drawing. Maurice Sendack's "Where the Wild Things Are" comes to mind, but Max's bedroom transforms over the course of 4 - 5 illustrations.
I think in your case, perhaps drawing the foot board and ghosting the castle over the foot board might help make the story a bit clear. Same with the hills at the front. Make those look like hills made from a blanket. Use every day objects you might find in a kids room to mimic the scene of a castle being stormed by a monster. A dresser and toy box side by side could be mountains in the distance
For the monster, you could have the family dog or cat ghosted as a dragon or some other mystical creature a kid might think up. Or even a little sister could be the "monster".
One way the monster would work as you have it now would be to have it be a shadow coming from the hallway door, with light coming from the hall, being your primary light source, with some ambient and reflected light filling in to help create the forms
I would crop much of the ceiling out of the frame so as to not have all the open space at the top.