Fergus drops a key which opens the door to the room n°15 on the map.
I don't know why it doesn't work for you !
I looked at a walkthru (a poorly written one) which seemed to indicate that the key was to the door at the east (it actually said west, but there is no door to the west so I assume the author just doesn't know his west from his east).
It's a lot of work to write all the soluce for the game or to make all the maps.
I have made a soluce for the demo then i stopped because i did not have enough time to continue.
Please don't say bad comments about the work of people who spent a lot of time to help the other gamers.
It's not nice at all.