I want to design a dungeon.

Let's see, for the first part, let's make it in the Outer Space. Right at the entrance, put on some Instant Deathtrap with infinite charges and cannot be disabled. The trap detection only works when inside the dungeon, so why didn't they put it right on the first square people step in?

The next Instant Deathtrap would be at the only entrance to the Rest Area.

The monsters will be robots that fire defense-ignoring lasers that bypass even immunity. The robots would be immune to physical and magical attacks. The only way to defeat them was by making the robots aim at themselves and they're basically aimbots.

Make the room the biggest possible and put it into an extremely annoying procedurally generated labyrinth. It will reset each time someone enters and exits the room, even if there's someone present, including monsters. Make the walls as high as the ceiling so flying wouldn't be possible.

Give the rewards to the adventurers in this proportion, on 10 chests:

5 are mimic.

4 are garbage.

1 is blank paper with 'Good Luck' written on it, just paper.

All look exactly the same.

There would be an abundance of chests nearby. If not opened and they walked past by, the mimic chest would instantly eat the adventurers. The garbage chest would explode, dealing massive explosion that didn't damage the walls at all the moment you walked past it. The paper with 'Good Luck' will summon mimics and explode (damaging only you) when you ignored it.

The mimics that wander around would act the same as monsters. Like the robots, the only way to defeat them was making it eat another mimic.

The position of the chests would be randomized, there is about 1 chest every walkway in the labyrinth.

Make the entire dungeon look the same. If one floor consists of three rooms, each of the rooms would be extremely complicated maze-like areas that auto-reset itself even when the auto-respawning robot monsters walk in.

Stairs to the next floor? Won't bother building it. On a side note, the entrance would also be sealed until you cleared the dungeon and you could revive infinitely in here. So, when you die you won't be able to return to the city.

Scroll of Town Portal? Warp? Mass Teleport? Well, it's the Outer Space.

Upon death, you'd only be teleported randomly around the same floor.

As for dungeon core, why must make it accessible to the adventurers? Just make the core room in some isolated dimension only you and you alone could access.

Last of all, make an undispellable trap in every dungeon entrance, first square, that will teleport every adventurer in every dungeon, right to the instant kill trap in this dungeon. The teleportation works one way only.

As a bonus, make the monster (robot) spawning location to be the Rest Area.