The Deeper You Go, the More Dangerous the Dungeon
Borrowing heavily from the earliest roleplaying games, dungeons are more dangerous the deeper down you go.
In pure old-school games, this was largely depth, and assumed a campaign centered on the dungeon. With more modular play, you'd end up with surface-adjacent complexes full of monsters far, far more dangerous than "level 1" would suggest by the rules. Still, the idea was codified around depth = danger.
In Felltower, it's a little more nuanced - the futher you are from surface-access points you are, the more dangerous it gets. So if you go down 5 levels, and then up 1-2 through a sub-level, it's likely more dangerous despite the more relative shallowness. Trying to game this out may work - PCs choosing to delve can assume the deeper into Felltower the riskier it gets, even if that means going up instead of down.
In game, followers of the Good God explain the the phenomenon by saying that the depth shade the denizens from the gaze of the Good God.
To really find dangerous foes, you must delve deeper. But the rewards are scaled to the depths, too, and tougher monsters are likely to have more treasure - the tables for monster and treasure placement in DF21 are exactly those used in Felltower. While there are occasional placed hoards on the upper levels, there are not many. Onc they're gone, Rule I suggests they won't be back soon . . .