I built a multi-level kobold warren that filled the interior of a large crag and was topped by a dragon's lair out of cardboard and Gaming Paper for my party back in our early days of 4e. A lot of carboard, a lot of guide-wire, and a lot of Gaming Paper. but in the end it was pretty cool, since the party could be split on multiple levels of the crag (they had to run from the base to the top to confront the dragon in a limited time period), and I could make it one enormous encounter.
We had the faster PCs running upwards 1-2 levels before the rest of the party, PCs dropping to negative HP and other PCs having to backtrack for them while fending off pestering kobolds, and a really malevolent green dragon waiting for the party at the top, ready to slaughter the wizard that was the party's only hope of escaping the island their ship wrecked off the coast of.
Good times. A lot of work, but totally worth it in the end. My players really seemed to enjoy it.
Sadly, I don't think any pictures exist of my (not so) masterpiece. =/