There is a version of this game where the second stage is a crypt full of skeletons and the third is a hell full of demons, because that is what the second and third stage of an action roguelike are full of. I built that version. It was fine. Nobody, including me, could have told you what it was about.
What fixed it was a rule that sounds like a joke and turned out to be a design document: the player is a cat, so every monster has to be a thing that actually frightens a cat.
What that rule does to a desert
Ask what really hunts a cat outdoors and you do not get skeletons. You get jackals that circle in a pack and charge together, and whose deaths make the rest of the pack howl and speed up. You get a vulture that wheels overhead at a distance and only commits when you are already hurt — it checks your health, which is a mechanic I would never have thought of and which the animal handed me for free. You get a scorpion, a sand viper that dives under the floor and erupts beneath you, a monitor lizard, a coyote alpha that rallies the others. Every one of those is a different movement pattern, and I did not have to invent a single one of them.
The Desert also has a tumbleweed, which does not chase you at all. It rolls, bounces off the mesas and off the edges of the screen, and sprays thorns on every bounce. It is the only monster in the game you cannot bait into a line, and it exists because tumbleweeds bounce.
And what it does to a house
Then you ask the same question about indoors, and the stage writes itself. The third stage of Catacrawl is called the Abyss and it is a house, seen from the floor.
- A haunted vacuum cleaner that drags you toward its nozzle. It has three attacks and all three come from being a vacuum: a constant pull, a long inhale that doubles it, and a bag that fills up with everything it has hoovered until the seams give and it coughs the lot back out as a poison cloud. The collecting is the fuse on the third attack.
- A spray bottle that squirts three times and retreats to recharge.
- A shower head that sprays an arc of water you can walk out of.
- Party balloons that float toward where you are walking rather than where you are, so running in a straight line delivers you to one. Turning is the counterplay.
- A man with a new year rocket, because the other thing that empties a house of cats is fireworks.
- A cucumber. If you know, you know.
Not one of those is a monster. They are objects from a kitchen and a bathroom, and they are frightening in this game for exactly the reason they are frightening to the animal.
The part I did not expect
The rule turned out to be a filter for mechanics, not just for art. A vacuum cleaner that drags you is a displacement attack. A vulture that waits for you to be weak is a conditional. A balloon that leads your movement is a punish for running in straight lines. I have never once had to ask "what should this monster do" — the object answers it, and the answer is always something the player can learn by watching rather than by dying.
It also settles arguments. When I was deciding what the Abyss's swarm enemy should be, the candidate was a dust bunny, and it moved in a serpentine weave because of how the code that drives puffball enemies wobbles. A serpentine weave is not what a dust bunny does. It is what a charging cable does when it slithers across the floor toward you on a sparking plug head. So that is what it is now, and when you kill one it shorts out and leaves a patch of floor arcing for three seconds — which means killing one on top of yourself is the mistake. The rule produced the monster, the movement and the counterplay in one step.
Where it stops
Honestly: the first stage. The Fairy Forest has toadstools, pixies, a bog toad, a thorn boar, a moss treant and an owl. The owl is a real predator of small cats and the boar is a real danger; the pixies are there because the first stage of a game has to be pretty and welcoming, and a fairy forest is prettier than a field of jackals. I let that one go.
Everything after it obeys the rule. If you are wondering whether the museum stage is full of things that frighten cats — it is full of things that frighten museums, which is a different joke, and one for another post.