The shelf idea is one I considered for my son before we claimed the loft for the purpose. The problems with this were :
- low window sill dictated the height of the whole thing. and meant that his existing bed wouldn't fit underneath.
- Fitted wardrobes (see below).
- Inward opening door needs something to prevent anyone from opening when the link section is in place.
- Pre-war house with chimney-breasts made for an awkward shape to follow the contours of the room.
None of the above are show-stoppers and if the loft hadn't been available, we would have gone ahead with this. The fitted wardrobes would actually have become a "feature" for the layout by having tunnel entrances in and out and a protected area within the wardrobe.
The other advantage of the shelf idea is that you can easily do the scenic work in sections - choose one side of the room for a station or whatever and leave the others bare until time and skills allow something more adventurous. These un-decorated sides can be of minimal width until they are developed further (which may help with planning permission from the Mrs...)
Above all else, have fun. That's what it's all about.
Adrian