The main entrance was built for sure (punched open with an airship, by the looks of it), but the cavern could easily have been there all along (and just accessed via Warp Pipe) - I doubt Bowser would have built in a Warp Star had he designed the whole place. Anyway, while there's a thunderstom in the Valley of Bowser somehow, the YI castle seems to be outside, but it doesn't fit the perpetually overcast depictions of Bowser's Castle's surroundings in most other games, so rather than Dark Land, I generally feel like it's another nearby island entirely that the Koopas abandon after the destruction of their castle.
quote author=SiFi link=topic=27164.msg1694517#msg1694517 date=1427456844]
Seeing another mention of "Dragon Koopas" has reminded me of another facecanon I have. Bowser and his family are Boom Booms, which are basically just hefty Koopas with ox-like muzzles and sometimes spikes and/or fire breath. Boom Booms are alternatively known as "Dinosaur Land Koopas" to differentiate them from Mushroom Kingdom Koopas, and that's why Bowser lived on Yoshi's Island during childhood and why Sumo Bros. and Football Charlies are also found in Dinosaur Land, because they're Boom Booms with specific skills.
read the above posts in more detail edit: shellcreepers bite, and my other headcanon is that "shellcreeper" is a racial slur for four-legged koopas
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Imo, there's too many differences between Boom Booms and Dragon-Koopas for them to be the same species, the most major being that Dragon-Koopas have soft belly scutes separated from their carapaces by regular scaled flesh, while Boom Booms (and the closely related Sumo Bro) have wraparound plastrons. They also lack the paired head horns, spiked tail and flexible shells of adult Dragon-Koopas, suggesting there's neoteny in their evolutionary line, making them resemble hornless, small-eyed juvenile Dragon-Koopas (although some youngsters already have horns - hence the other neotenic species, Koopa Kids, have horns too). Sumo Bro are of the same line as Boom Booms (they're definitely sister species), but their single head horn and lightning/fire abilities set them apart from the transformation-magic-specializing Boom Booms (although as seen in Pom Pom, Boom, Booms can have smooth shells like Sumo bros - and unlike Dragon-Koopas).
I still like the idea that "turtle" is a derogatory term for Koopas. I consider Shellcreepers their own species of four-legged colour-changing Koopa - the precursors to Koopeleons, and the descendants of four-legged Koopa Troopas.