In the land of Aviontix there are ruins. A lot of ruins. There are many sand buried cities, king’s tombs (They were a very religious people, focused upon their leaving to the afterlife), and huge statuary. However, Aviontix was a kingdom so long ago, that everything has been looted. (Figure there has been a whole historical epoch or two since then) There might be a hidden tomb or something somewhere in the lands, but it is unlikely. This does not stop people from selling maps to them all the time.
You come upon a ruined building in the back section of a city park (or other out of the way area of the city). The ruins are fairly overgrown. All that is really standing is a doorway and its frame. If you pass through the opened door, you travel to a different world. If the door closes, there may not be a doorway back to get you back.