Do you have a spare tank?
LOOK PEOPLE!!! - EXAMPLE OF WHY YOU NEED A QUARANTINE/HOSPITAL TANK!
Ok back to this.
If you are certain the fish cannot come out and is still alive (if dead you should remove the ornament and dislodge the body or it will reak havoc with your water), you cannot leave it there.
It is probably already quite stressed out but I'm going to suggest you try prodding its (gently) tail to force it to try to swim out. Keep the ornament on its side like in the picture so he can swim forwards. Once he has, he will almost definately have injuries and it's essential he is in excellent water quality with dimmed lights, anti-bacterial/fungus medication (or some aquarium salt if that's all you have) and as few other fish to be harassed by as possible so as to minimize stress. He will be especialy susceptible to disease for a while so pamper him and watch him carefuly for early signs.
I realy can't see any other options. You couldn't take the ornament out and smash it as you would almost certainly kill the fish.
You can try lifting the ornament to the water's surface in stead of prodding but either way he will likely come out with some scratches.
The only other option would be to wait and hope he dis-lodges himself with minimal injury but I'm afraid he may simply die in there if left.
As a last resort, do smash the top of the castle and hope he can thn escape or the bottom cracks.
I'd also encourage you to remove the ornament afterwards as fish tend to have an uncanny ability of getting into the same messy situation more than once.