Must be a ferret nation or another type of cage as even tiny baby rats are fine in a critter nation.
If it is a ferret nation, some people cover the outside of the cage with hardware cloth so that small rats can not get out.
Hopefully your cage is powder coated (paint is baked onto the wire) because galvanized metal will absorb urine and soon stink
If you have small levels, I would suggest using a dollar store dish drain tray, cut to fit and cable/zip tied on
For larger levels I would suggest a large piece of thick linoleum cut to fit. The thick linoleum provides enough padding that wires can not be felt through it. You can purchase it from flooring or hardware stores and often they may have left over sections market down for sale.
Both of these surfaces can be cut to fit, are easily wiped off, easy to replace if needed, and they do not have smooth surfaces so rats have traction.
previous suggestions are good, however stick on tiles will be a mess to try to remove if you ever need to and I imagine urine will get between the tiles; I think that carpet will likely soon become very stinky from urine and feces.
Here are two pictures that show what I mean ..... the first shows two martins R649s I had connected in 2005 with thick linoleum on the large levels and dish drain trays on the small balconies
the other is Laurel's sons Paul, Rubarb, Casper, Luke, and John in a pretty bare cage
in 2008 and shows two levels covered with thick linoleum attached with cable ties