Tascha,
I read the walkthrough and you have to do all the things stated on level 18 before you reach this part of the walkthrough:
Left door
Look on the right, there's a big golden coin on the pedestal that is closest to the dor. Pick it up - it is a disc with instructions. Go to the courtyard and click on the stone structure in the middle.
This just above the picture of it at end of the wt.