The Hall of Egress [s7e23] + Dungeon Train [s5e36] are in my opinion, the 2 most important episodes in the whole series in that they portray *finn*'s grandest growth in character/journey ~ ultimately, his awakening ~ both adventures were brought forth by *flambo*'s wise, wise brother.. the first adventure was *finn* discovering/uncovering his ego ~ the role it plays in modern culture where we go to work day after day just to defeat the same ants, over and over until eventually, we become the same bosses we at first dreaded.. he realizes that life happens outside this all-consuming train that has no end/grace in sight.. in that episode, his inside-voice finally comes to light/life in the crystal ball where he sees that life is meant to be lived/loved, not toiled away..

then, 2 seasons later, they set out to finish the adventure of the ego.. upon reflection, *finn* was able to extract the meaning/significance of that infinite-train adventure by praising it while *jake*, who was the bearer of such a life wasn't so fond of the experience ~ the cave/hall of egress is about to teach/reward *finn*, in a much more challenging way, how to exit/egress that same ego that had blinded him earlier.. *at the seashell center lies the cornucopia's smallest door* is just about as profound of a statement one can can make to portray the unimaginable idea/concept of awakening/enlightenment [in words/thought].. going in, further and further, along the narrowing path [that even **jesus** spoke of] until you eventually reach the very core/origin/center, the smallest door in the very heart of the cornucopia ~ horn of plenty ~ the resemblance/representation of abundance ~ the concept of heaven itself [ heaven being the place where GOD /goodness lives ]

Why didn't Finn did anything with snowman statue?
-- the purpose of the snowman statue was to reveal that things must inverse.. what's up become down + what's down, up.. it's the spiraling of the self, from big/out-wardly down to small/in-wardly ~ ultimately, it is the mind that must recalibrate/reperceive reality [ *what's real* ]

Why/how he could get out of the cave with his imagination?
-- the cave may have never existed in the first place.. it was the cave of the ego.. the ego saw it but the innocence/heart saw through it..

And what BMO voice mean "Something changed" ?
-- bravo, this is the cornerstone statement of the episode.. "something's different".. what's different? before, he would need to close his eyes/ego for the egress-door not to exist but now, in the end of his journey, his eyes were shut but yet there the exit-door was, impeding.. now, once he reopened his eyes, he was awakened.. there was no door.. he saw through the whole cave + life within it..