There are 6 different Environments in the Game. Each one has a specific Background with its own Monsters.
The Environments repeat every 100 Levels, for example: Dungeon Level 101 would be Catacombs again, Dungeon Level 341 Water Temple.
To see which Monster appears in which Environment, see the Bestiary on the Compendium.
Catacombs:
The Catacombs is the first Environment. It ranges from Dungeon Level 1-20.
Fire Caverns:
The Fire Caverns is the second Environment. It ranges from Dungeon Level 21-40.
Water Temple:
The Water Temple is the third Environment. It ranges from Dungeon Level 41-60.
Frozen City:
The Frozen City is the fourth Environment. It ranges from Dungeon Level 61-80.
Ascension:
The Ascension is the fifth Environment. It ranges from Dungeon Level 81-99.
Top of the Dungeon:
The Top of the Dungeon is the Boss Environment. It appears every 100th Dungeon Level.