You have to get the key so you can open the floor's door.
Here you are, the door open to the next floor.
At Floor 2 you'll have to fight those black slimes, which are faster than the green ones.
At Floor 3, other knights show up. You can kill them only from behind.
Here you've found a treasure chest, which contains special items.
You'll find a wizard at Floor 4, who throws lightning bolts at you.