Rats are very active and like climbing so need the height of their cage to be filled with hammocks, tunnels, bridges and ropes. Try to make sure there's no large gaps of space in the height, make use the of the whole space.
They also like to have houses and tunnels on the ground level. We don't recommend using fleece for the floor as rats love digging and foraging through substrate.
Some examples of things that shouldn't be used