Some things are basically the laws and rules of the universe for Felltower.
They're not in-world rules but they do describe how the world works.
Monsters Replenish Faster than Loot
Loot is the name of the game in Felltower. The dungeons are rich with treasure, but are also monster infested and trap encrusted. As treasure is taken out, it is sometimes replenished by new visitors bringing in loot as they come in. Others find it a convenient place to stash what they've found.
But it's at a fraction of the pace of the replenishment of monsters. Nature, and Felltower, and GMs, abhor a vaccum. As the PCs clear out areas of fierce creatures, lesser creatures will be able to roam further. Vermin that served as prey - or just sport - for the fiercer types will spread. As monsters that serve as a buffer between territories of other monsters get killed off, conflicts will arise that cause monsters to move or reinforce areas themselves. Monsters will breed - and ones that have been kept out, pared back, or otherwise held at bay by intelligent types will multiply once the intelligent types have been wiped out.
In other words, monsters multiply.
Treasure, though, does not.
The game requires that you outpace the replenishment of monsters if you want get more loot. You can explore and re-explore level 1, and always find things to fight . . . but they're mostly time and resource killers. The dungeon always beckons you deeper if you want more loot. It's less and less profitable to explore the "cleared" areas. Yet is never really, truly safe to explore the "cleared" areas.