Not to mention there are dark colored faces of floating Bowser heads that randomly appear laughing demonically in there.
What makes this even worse are the implications. This isn't your goofy, comic relief Bowser. Outwardly, he just seems a bumbling powerhouse, but now that you're in his psyche, where all is lain bare? Bowser is genuinely twisted and malicious as he claims to be. His mindscape is warped with visions of flowing magma, cannons laying waste to the landscape, irons chains wrapped over the very ether of the plane, cracked and decrepit brick walls locking everything in, sharp and dangerous protrusions everywhere. In hindsight, the Zeekeeper was dead-on before.
Zeekeeper: This Bowser guy sounds like another nutjob.
It also make the implications of this earlier scene worse. After summoning Neo Bowser Castle, Bowser and Antasma test its power by firing on nearby islands. Now if we assume that the islands were inhabited, and that Bowser is as genuinely malevolent as his dream makes him out to be, then we've just seen him possibly murder god knows how many people onscreen! So much for Villain Decay.
Yeah about Neo Bowser Castle. Those "vines" running all over the place transmitting dream energy from Bowser to power the castle look a little too much like purple veins to be comfortable, especially when you get to Bowser's bedroom where they all converge like a heart.
Dream's Deep. It's like Bowser's Dream above, but different. The entire place is a bizarre garble of purple and green with neon Luigi faces floating in the background. Spinning and running Luigi holograms run all over the screen. And it's all accompanied with an eerie piece with tribal-sounding percussion.
The first time through, you're briefly separated from Luigi. There are quotes floating around the screen and in one segment there are multiple Luigi holograms that follow you, only to vanish when you turn around. To make things creepier, this is the deepest part of Luigi's subconscious. Some of the quotes are profoundly thought-provoking and give a lot of character to Luigi.
"Don't leave me behind, bro!"
"I'm not clumsy!"
"Big Bro! Big Bro!"
Two words: Dreamy Bowser. Bowser inhales the fragments of the Dream Stone and gains a few Reality Warper powers. Now Bowser is already powerful on his own. Who knows what would have happened if the Mario Bros lost?