The blackboard of the middle ages
A large steel shield in the form of a sea-shell attributed to Athena, the goddess of civilization, craft wisdom and war.
A Castle or better a fortified tower is floating over the country side and being attacked by goblins (or other monsters, depending on PC strength)
In the middle of the sky over the land of Dankij there is a door. The door is fixed in an upright position and appears to be fastened to the very sky itself. Worldwide rumors say it has been there since The Creation. What could be behind this door?