C64 version, level 17. There are two ways to get in the exit door. One is the long way, and the other is very short. Unfortunatly, neither are the answer to the secret level. Maybe a bug? (I have not tried the Amiga version yet.)
See the VIPER's retrogame videos comment below and follow the YOUTUBE video to find the answer to the secret level. Nice homage to Tenebra and Tenebra 2 for the exit to the secret level. I would have never found it without the comment in VIPER's video.