Hussar
Legend
If I was an Adult Red Dragon my lair would be close to or inside a magma pit at the base of a volcano. I would have used Efreeti slaves (probably by giving a high level caster a favour) to forge a protective treasure room off to one side so as not to be destroyed by the magma every time the volcano erupts. To actually reach the inner core of my lair, you'd have no choice but to fly over swim through magma.
See, to me, I have no interest in running a dragon like that. Bleachhh. It's about as far from a mythical dragon as you can get. What's the difference between your dragon and, say, a high level wizard or virtually any other BBEG? A dragon doing a favour? Really? I want my dragons to be like Smaug or the dragon from Dragonslayer - engines of destruction. I do not want another moustache twirling villain.
I mean, heck, Smaug didn't build his lair, he just killed the inhabitants. I'm actually drawing a blank on any inspirational fiction where the dragon goes out and hires contractors to build his lair for him.
To me, that's about as exciting as watching paint dry.