Its makers started building levels by hand, but with a team that small, lead designer Sebastien Benard wrote in 2017, they would never have had the time to do it properly. So they let the computer generate everything, and the levels came out, in his words, "illogical, chaotic".

The fix: people make the rooms, the computer only shuffles them. Every room is built by hand, with its doors and its job: a fight, a treasure, a shop. Then a plan for each part of the castle: the entrance and the exit first, then the special rooms, then the rooms you fight through between them. Only then does the computer go: for each spot in the plan it tries a random room, and if the doors don't fit, it tries another. Even the monsters come by the tile: say, one for every five tiles of fighting room. In his words, "the key for us was in having the algorithm involved in the most restrained way possible."

The castle, rooms, plan and monsters in this film are our own simplified version, built in code. Not affiliated with or endorsed by Motion Twin or Evil Empire.

Which game's level generator should we open up next?

Sources: Sebastien Benard, "Building the Level Design of a procedurally generated Metroidvania: a hybrid approach", Game Developer (Gamasutra), 29 March 2017.
Sounds: our own, made in code.