The Fifth floor was one of the many floors which weren't ever stepped on. In this case the sentence becomes more literal.

  Until the Only One barged in, nothing stepped on it as the Boss monster was very territorial and killed off anything that wasn't some type of plant and even then, was particularly picky.

  Usually the middle boss on the 5th dungeon floor was the most benign as to leave more room for progress during the building of other floors.

  After all they truly weren't "middle" Bosses as they appeared every 5 floors, but the adjective spread due to the Maripod dungeon that insisted on such terminology.

  The Core couldn't comprehend why mere words were that important but it adjusted to the manual.

  Besides, its only middle Boss was actually a middle boss since it only had 10 floors.

  At first it wanted to make it as "harmless" as it could, to allow for more creativity in later stages where it could unlock more monsters, unfortunately that dream died slowly during the time where it patiently waited for adventures to arrive. Instead it decided to just move its middle boss lower as it gained more floors and make one that was as benign as a dungeon monster could be to place in its stead.

  That thought also led to it getting creative with its current floors. If it didn't have to worry later on about sudden changes in difficulty level leading to S-rank adventures why not go all out? Although it might not seem like it at first glance but that was because The Core wasn't the most creative dungeon core. In fact most of its floor designs were recreation of themes that it found in the manual.

  This lack of creativity also led to quick boredom that led to overhauling its entire dungeon. A feat done by no other core. After all, every other dungeon had adventure's coming knocking after a week maximum. It was the only one unfortunate enough to be born in the middle of nowhere.

  But its dungeon monsters had no such problem, they just didn't do it due to their lack of intelligence, not creativity. Although those do tend to come hand in hand.

  As such its middle boss, a Foliage Shaper, a creature of excessive creativity, although not that smart, remodeld the entire fifth floor of the dungeon with no objection from the actual owner.

  She even redid her home floor when asked to! Although The Core only found that out on Foliage Shaper number 6.

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  Because of her remodeling, the entire floor was under her purview, especially the floor, as it was absolutely coated with vines upon vines. The Trees, although also covered, were allowed to live, unlike the grass, as such it was much safer to traverse on the treetops as they were less supervised.

  Not that The Only One came up with that, it just started cursing as soon as it stepped on the vines and found its feet sinking to the ground, which was hidden about 30 centimetres under the outermost vines.

  It began thrashing wildly instead of firmly planting the other foot, the one that wasn't being tightly bound by vines, on the ground and using the leverage of the nearest branches to hull its trapped limb out.

  The Foliage Shaper wasn't known for its strength or endurance but rather lifeforce siphoning power. Which is also where it got its name from, you can easily shape other plants into desired positions if you can simply take away their lifeforce if they aren't in it. By process of elimination you will be left with The Foliage that you liked.

  Taking this into account it is no surprise that, even with such idiotic distribution of force, the vines got destroyed.

  Not that it mattered to The Foliage shaper, she was so huge and had so much condensed lifeforce stored in herself that such destruction of the vines had no impact on it whatsoever. Not to mention the emergency lifeforce she could extract from the trees if things truly got bad.

  The Only One put his other leg on the hole he created, so when the middle boss's vines covered the invader again, it got immobilized more than before. Not that that helped much, even as The Foliage shaper tried to climb up to restrain it further, she just couldn't do it fast enough.

  The Human once again destroyed the vines covering it.

  The Life-force syphoned didn't cover the life-force lost either, and so the fight was looking quite bad for the boss of the floor.

  The Core could only hope that the stamina drain would make the subsequent fights easier for its other monsters. After all, the door to the next floor revealed itself only when the middle boss was defeated and that would take a while due to her impressive regeneration abilities and sheer volume.

  With each step the interloper took, the Boss would syphon a bit of lifeforce and then lose more than thrice the amount of energy it took to do so.

  But The Core noticed that the human was becoming much more frantic, even manic in its quest to find the entrance to the next layer of the dungeon.

  It teared through the vines with reckless abandon, not caring that it was exposing itself. Not that there was anything to expose itself to, which was something that The Core would need to change. Add some monster birds, maybe? But will the middle boss be willing to tolerate anything that isn't some form of plant life?

  It also saw how its middle boss became more elated, putting more lifeforce into the vines so that they could reach the invader sooner just to syphon a bit more Lifeforce in exchange for losing so much more.

  With the knowledge that it couldn't see the Lifeforce of outsiders, unlike the only monster of the 5th floor, it came to the conclusion that seemed quite fantastical.

  The Only One didn't have much lifeforce left.