Quote: "It says 15x15x5, does that mean we can use 5 layers?
Also, what is the deadline to this contest?"
How about reading what it actually says on the competition site?
Both answers to your questions are on the first damned page! Lazy arse
Quote: "So does that mean it can only be one room? And does it have to be just one floor? How meny floors can we have?"
It can be as many rooms as you can fit into a 15x15x5 sized level. 5 being the vertical (i.e. 5 layers up/down maximum).
"Bite my shiny metal ass" (Futurama)
"Don't ping my cheese with your bandwidth" (Dilbert)