I like to go with a motiff. Making the dragon a crafter could explain the Iron Golems, and alot of other nasty possibilities.
You could also make him a summoner. Imagine Fire Elementals, or Earth Elementals. They could be protected from fire with spells, but also have them take 9/10ths cover in the ground. Big boost to AC and saves, possible no AoE damage due to cover. Can only attack grounded PC's, tough.
Steam vents, smoke, and toxic gases. Fields of volcanic glass. Effectivley caltrops, which he would be immune to due to DR. Add large amounts of "soft" bedding material like branches and leaves, which are replaced reularly by servants. Put these together with one blast of breath and a flurry of wing beats, and you have the scariest enviroment imaginable.
Pits filled with water (steam fodder), spikes, or for the truely evil, grey ooze. Sprinkle liberally with pumice and a good Glammer spell, and it is now all but invisible and fits the surroundings perfectly.
A glaze of metal with a low melting point. After one breath, you get all the goodness of a Heat Metal and an Entangle spell, once it cools. If you want to get really meticules, you could spike it for a caltrop effect till melted.
Just off the top of my head.
