So there is a pair of opto's that are at the entrance to the drawbridge. So if the drawbridge lowers you know it isn't an opto issue there. Next there is an opto set right in front of the gate. Test that opto pair in the switch diagnostics.

Test the optos, as exflexer said. Also, test the switch in the green moat trough, under the castle. If those switches are working intermittently, it wouldn't give you an error. If picks up errors when it detects a switch that hasn't been triggered in several plays.

Most likely the moat switch works because the drawbridge will lower. The upper moat optos will only award you a couple hits before then the CPU is waiting for the lower moat switch to be activated.

Don't forget the swinging castle door on the left - balls goes to the moat there too. The game knows if it came there or through the gate because of optos in front of gate. No optos on the swinging door.

LTG

There is a rollover switch that is in that portion of the moat. That is how the game knows you successfully made the castle lock shot.