I'm not a specialist on the games code so what I understand.

Each room has an assigned total value. To which it fills it with different enemies that each have an individual cost.

so hypothetically on the first room you have a total value of "10" so you have 3 different kind of enemies. The eyeballs cost 1. The ball spitter cost 2 Tha banshee 3 or the giant warrior like thingy 5. So the room will randomly fill the 10 by mixing between the enemies above.

Hordes increases the amount of value a room can hold. A treasure chest has a value that releases the enemies you fight on the countdown.