"......How was it?"
A full hour. I had spent the entire time giving a lengthy explanation of the future I envisioned for the Demon Lord's Castle.
Lapis and Laura fell silent, lost in thought. After a moment, both of them spoke.
"This is absurd."
"Ah. A pipe dream."
"Do you have any idea how much money this would require? Just repairing the highway alone..."
"More than anything, it's far too dangerous. Instead of intercepting adventurers, you want to draw them in? One misstep and your life will be at risk."
I grinned.
"But isn't it fun?"
The two of them fell silent again, lost in thought. Lapis spoke in a tone that suggested they take things one step at a time.
"Let's set aside the outrageous plan for now. Let's start with the practical matters and work through them one by one. Lord Dantalion, you mentioned you intend to extensively rebuild the Demon Lord's Castle."
"That's right."
My Demon Lord's Castle was simply too shabby. It was practically just a cave. It was absurd. I had absolutely no desire to live in some grand palace or anything like that, but at the very least, it needed to have the facilities for a monster army to conduct a proper defense.
The final scale of the Demon Lord's Castle I had in mind was...
"......Ten underground floors, no matter how you look at it? I've never even heard of a scale like that."
Lapis was stunned.
Ten underground floors of dungeon!
The cave housing my Demon Lord's Castle was enormous. It took hours of walking just to get from the entrance to the Demon Lord's chamber. Because of that size, we only ever lived near the chamber itself. At most, we'd make the trek to a somewhat secluded underground pool when we needed to wash.
Expanding a cave of that magnitude to ten floors' worth of space. From Lapis's perspective, it must have been an unbelievable proposition.
"It's only the final form that has ten floors. We have to start preparing from the first floor. In any case, the Demon Lord's Castle needs to be much larger than it is now."
My plan is simple.
To let monsters flood into the Demon Lord's Castle without limit.
I realized this after coming to this world: residing within the Demon Lord's Castle is an incredible honor for any monster. It's not merely about living near the Demon Lord. To put it plainly, the castle itself is a heaven-like environment for monsters.
What maintains the bodies of monsters like Skeleton Soldiers or Zombies—creatures that should normally lose their vitality immediately and collapse into bones and corpses—is none other than mana.
Mana drifts everywhere in nature. However, to ingest enough mana to sustain one's body, you need to hunt and consume creatures at least as substantial as animals. The closer a species is to the top of the food chain, the more mana it accumulates within itself.
This is why humans are the most tempting prey for monsters. Humans, who have consumed all manner of flora and fauna throughout their lives, are essentially walking clumps of mana compared to other species. Humans, who can live without spending mana; monsters, who must spend mana to survive... conflict between the two races is inevitable.
But what if a monster is inside the Demon Lord's Castle?
The Demon Lord's Castle is overwhelmingly rich in mana year-round. It's a place where simply mining ore from the cave walls yields Mana Stones. Once a monster lives in the castle, it no longer needs to go to the trouble of hunting humans. Just breathing calmly is enough to more or less solve the problem of survival!
If the Demon Lord did not hold absolute command over the monsters, they would have long ago plundered the castle and settled in. It's that desirable a nesting ground.
—And I'm opening it up.
Goblins, Orcs, Lizardmen, Zombies, it doesn't matter. I will admit them into my Dungeon without discrimination as to race.
No qualifications are needed.
There's no need to become my subordinate.
They just need to bring their bodies.
"All other Demon Lords aim for a small, elite force. The Demon Lord's Castle is a treasury of mana. Only the most powerful should enjoy its treasures... That's an exceedingly plausible line of logic, but it doesn't apply to my Demon Lord's Castle."
I brought my palms together and smiled.
"We rely on sheer numbers. We advance using only overwhelming force."
We will construct a great labyrinth of ten floors. We will pack every single one of those floors chock-full of monsters. And I myself will place the Demon Lord's room on the tenth underground floor, the very bottom level, and live there at leisure and in peace. It's truly a brilliant solution, save for the point that the construction costs would be astronomical.
Lapis groaned.
"The dignity of a Demon Lord's Castle…"
"Dignity, my foot. The seventy-first ranked Demon Lord is going to get himself killed chasing dignity."
"My Lord. You spoke of using human wave tactics, but there's a major flaw in that approach."
Laura was burning with determination to shut down my plan no matter what. Perhaps she, too, preferred a small elite force over a large but low-quality army. Man, tacticians are a pain like this. Regardless of the era, they all harbour a romantic notion about elite squads.
"Go on, then."
"If we go with the numbers strategy, most enemies wouldn't even dream of infiltrating the Demon Lord's Castle. But what happens if the other side brings an incredibly powerful elite squad?"
Laura spoke with a confident tone, as if to say there was nothing that could be done about this one.
"It's such a massive Demon Lord's Castle. There's no need to take down every single monster. All they have to do is break through the shortest path from the Castle entrance to the Demon Lord's room. They'll aim for a quick and decisive battle. On the other hand, we've got nothing but rabble. If an overwhelmingly powerful enemy shows up, they'll all flee to save their own skins. The Castle will fall in no time."
"Heh heh."
I laughed.
"That's exactly it, Laura. That's why we need a major reconstruction."
"Hm?"
"I'm going to turn the entire Demon Lord's Castle into one enormous labyrinth."
Laura tilted her head. From her perspective, the word "labyrinth" had come out of nowhere in the middle of a discussion about the Demon Lord's Castle. But to me, building a dungeon in the form of a labyrinth was the obvious choice.
It was to restrict the adventurers' movement paths.
"First, the movement paths would look something like this."
I sketched a rough blueprint on the ground with chalk.
────────────────────
Dungeon Entrance (Floor 1) ―― Floor B1 Entrance ―― Floor B2 Entrance ―― Floor B3 Entrance … Floor B10 Entrance ―― Demon Lord's Room
────────────────────
"At this point, the entire route is enclosed on both sides by solid walls."
"Oh. So that's why you called it a labyrinth."
I nodded.
"The walls need to be made as thick and sturdy as possible so there's no possibility of boring through them. The condition here is that the distance between each entrance can't be too far. For example."
────────────────────
Floor B1 Entrance ―――――――――――――――――――――― Floor B2 Entrance
────────────────────
"If you weave the maze back and forth like this, making adventurers trudge around an entire floor, they'll decide it's more efficient to just punch through the walls. They won't be able to break through all the walls in just a few days, so they'll start gradually claiming ground from the entrance outward. That can't be allowed."
Too short and it's pointless. The adventurers would take virtually no damage.
Too long and it's pointless. The adventurers would just smash through the painstakingly constructed maze walls.
"It needs to be moderately long—long enough that when they think about speeding things up by cutting through the walls, they'll realize it'd be absolute hell. And there's more. We also have to account for the monsters' living quarters."
"Don't the monsters live inside the maze corridors?"
I shook my head.
"The corridors will be quite narrow. They'll vary by section, of course. But regardless, large numbers of monsters can't inhabit spaces as tight as corridors. So the monsters' residential areas need to be set up on the other side of the maze walls."
I added more to the drawing on the ground.
────────────────────
□ □ □ □ □ □ □ □ □ □ □
■■■■■■■■■■■■■■■■■■■■■■
Floor B1 Entrance ―――――――――― Floor B2 Entrance
■■■■■■■■■■■■■■■■■■■■■■
□ □ □ □ □ □ □ □ □ □ □
■: Dungeon walls
□: Monster habitation zones
────────────────────
Raora stared at my blueprint and shook her head, clearly thinking this wouldn't do.
"Wouldn't the adventuring party and the monsters be completely separated like this! There wouldn't even be a fight."
"Oh, come on, Raora. This is just a rough sketch for convenience's sake. You have to arrange the corridors properly. You need to make sure that from the monsters' perspective, the areas they're required to guard are spread out and passed through in various spots."
"Hmm."
If done that way, fights would only occur in specific alleyways.
I made a few small adjustments to the blueprint.
────────────────────
□ □ □ □ □ □ □ □ □ □ □
■■■■■■■■■■ ■■■■■■■■■■■
Floor B1 Entrance ――― (☆) ――― Floor B2 Entrance
■■■■■■■■■■ ■■■■■■■■■■■
□ □ □ □ □ □ □ □ □ □ □
■: Dungeon walls
□: Monster habitation zones
☆: Combat Zone
────────────────────
Whenever adventurers passed through here, they were bound to end up in a fight. From their perspective, it would look like monsters just kept spawning in corridors that appeared completely empty. Of course, I'd have to make sure they never discovered the gaps in the dungeon walls, which meant setting up various contraptions.
This was what you'd call a respawn zone.
"For instance, we could cultivate mushrooms on a massive scale in this area. The mushrooms would absorb the mana inside the dungeon as they grow. The ambient mana would drop, and the monsters would have to eat the mushrooms just to survive. From the adventurers' perspective, mushrooms that have accumulated a good amount of mana are worth money. …The monsters would fight to defend this place, and the adventurers would fight to raid it."
Even I had to admit it was a vicious scheme.
If left alone, countless monsters could survive simply by breathing—perhaps not in luxury, but they'd get by. But once you started cultivating something like mushrooms, that option vanished. They'd have no choice but to fight the adventurers.
"They're still just adventurers, though. Is it more efficient to keep living in the wild and raid human settlements, or is it more efficient to live in the Demon Lord's Castle and occasionally face a dozen or so adventurers? I'm curious to see which choice they'll make."
Moreover, I added.
"The mana density also varies by floor. In this cave, the deeper you go, the thicker the mana becomes. Monsters that prove themselves by fending off adventurers will be granted the right to relocate. If you want to live well, fight well. That ought to be pretty motivating, wouldn't you say?"
"……."
Laura nodded with a hum, her expression vaguely dissatisfied.
"Then by the time we reach sub-level ten, only elite monsters would remain."
"That's a story for the distant future. We start at sub-level one—just a single floor. I need to test whether things are going the way I planned in various respects."
That was when Lapis cut into the conversation.
"Dantalion. We need to address the prerequisite first."
"Hm?"
"The money."
Lapis spoke in a very serious tone.
"You call it a 'wall' as though it's a simple matter, but we're talking about laying out labyrinthine corridors in a space of this magnitude. That's an enormous construction project. A couple of coins won't even come close. Where exactly do you intend to get that kind of money?"
"Oh, that?"
I replied casually.
"I'm going to borrow it."
"……What?"
"Debt — don't you know what debt is? I'm gonna rack up a nice big loan."