The 12th floor was going to be ocean themed, which meant that The Core could comfortably dig a hole on the bottom of the 11th floor, right at the end and call it an entrance.

  Of course, it also had to make sure no sand would fall through as that would make it less of a hole and more of a depression, ignoring the fact that since it didn't currently connect to anything it already was one.

  During this endeavour The Core even considered making it a crevice instead of a hole but quickly dismissed it as although invaders squeeze through small spaces would be a good trap for they would be vulnerable during entering and exit such a crevice it also increased the risk of them choosing to retreat and thus not dying in its halls.

  When it was done with the entrance it began carving out the ocean floor, a time intensive undertaking as it planned to make the 12th floor truly massive.

  More so than initially planned in large parts to the Dieflig Squids but the Oceanic Hydra and Ornamental Kraken also played a part.

  During the digging process it realised the fact that water was pouring through to the 12th floor from the 11th meant that its volume on the latter was decreasing so it quickly made a runic circle on the stone beneath the sand near the drop, stopping the water movement.

  Then it refilled the 11th floor until it was back to its previous volume and returned to working on its next floor.

  It took much more time than the previous floors but The Core was very happy with the result, even if said outcome was just an empty room with no hiding space.

  Thinking about what to fill the empty floor with to make it look more like the ocean floor made it realise that not having a light source would be great as it would decrease the visibility for the invaders even further.

  Which then made it remember it didn't put a light source on its 11th floor.

  A mistake it made because it didn't see through eyes like an invader and therefore was aware of light and darkness only through the manual and the memories it absorbed rather than experiencing them first hand.

  It quickly rectified the mistake through writing runes in the ceiling of the beach floor to illuminate it.

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  Then it realised that if only the 12th floor wouldn't have lights then the entrance to 13th floor would be like a glowing beacon to the Invaders, decreasing the difficulty and defeating the whole point of having a no light room.

  The Core thus could only discard the idea before going back to working on adding things to the 12th floors, like crevices in the walls that didn't lead anywhere.

  It then proceeded to fill the floor with water but stopped when the volume reached the waters of the 11th floor.

  Both the liquids were salt water so mixing them up wasn't a concern that it would be if one of them was another liquid or fresh water.

  That thought made The Core want to make an acid lake at some point.

  It focused back on its 12th floor and made a drifting cave in the middle of it, although given that it could move that was likely going to change as soon as it set up water currents, and filled it with one singular chest that contained a sword it deemed of good quality and coins.

  The Core figured since most of its invaders use a sword as a weapon of choice then using it as a reward would lead to better results than if the chest were filled with axes or spears.

  With that done it also spread a thick layer of sand on the bottom and created fully grown algae on it that already reached the top of the floor.

  This would impede the cave's movements but not by much.

  The Core then realised that this would instead impede this floor's monsters movements so it cut down on the amount of algae at the back.

  It then made water currents using runes carved on the stone walls and those on the floor beneath the sand.

  After a bit of thought it also made one on the ceiling that was stronger than the rest, located further in the back, near where the planned exit would be.

  With the floor done it began creating monsters that would populate it.

  Starting of course with the shiny, new options that it resisted picking for the previous floor because of thematic divergences.

  That and if it put monsters into environments they weren't compatible with their fighting prowess would decrease even if they wouldn't die.

  It quickly created a dozen Dieflig squids and placed them within the algae, not their usual hunting grounds but how bad could it be? It was sure they could adapt.

  Then the Oceanic Hydra went to the back of the floor, where it could swim around comfortably.

  And finally the Ornamental Kraken went into the cave with the treasure.

  It was small, about the size of a fish but it was still called a kraken for its impressive physical might that didn't seem to fit its stature.

  The Core also added more monster fishes to further harass the invader and came upon a glowing one in its monster catalogue.

  Which obviously meant that the no-light floor plan was back on since now the Invaders wouldn't be able to tell if a beacon of light was a threat or not.

  And if they encountered the Luminous Deep-Eel before they saw the exit they would be much more likely to not investigate it!

  Which The Core was sure was gonna happen since it created the Luminous Deep-Eel practically everywhere it could on this floor without making it seem crowded.

  Seeing its new floor filled with satisfaction but it didn't waste any more time admiring it before going to make another one.