You can
- play up to level 3 behind the first goldkey-door and trigger the first bossdoor-switch on the way in level 2
- then go back to level 1 take the teleporter there to level 3, open the secret room with treasures and the alternative first bossdoor-switch
- after that play onward to the second bossdoor-switch
- and finally open the endboss-door without having triggered the third bossdoor-switch which is behind the golddoor near bossdoor. Only 2 of the three runes at the door will be lit. Seems like the game just counts how many switches have been pressed (3) and not which ones.
However it would be a bit sad if that option to make some money by going all the way back to the teleporter and opening the treasure-room would be prohibited as i consider it sort of a secret/tactic.
Maybe just change the bossdoor-trigger-system.
