I had the same thing happen on mine. Frustating that you can't tighten the scews while the door is slid over the motor spindle or if you can I could't figure it out. only delay I had was getting the back nut off the wire ramp that launches the ball up to the castle. Went and bought a longer nut driver just to be extra careful. I used a tad of lock tight so hopefully won't happen again.
Quoted from GRB1959:
I had the exact same issue with my ECWOZLE a couple of months ago. You will need to remove the castle playfield to get at the mechanism underneath it to tighten up both set screws for this single castle door.
It took me about 45 minutes from start to finish and I was extremely methodical when I removed the castle playfield as I didn't want to mess anything up. The manual contains instructions for removing the castle playfield.