If you have a look at the cage on the link below - there is a joining shelf at the back and a long wide ladder. You put the ladder across the two long shelves butting up to the joining shelf. That more or less makes a full level - the photo above shows how you could add another piece of wood or even stiff cardboard, or a flat hammock across any remaining space and just leave a narrow gap at the front (or back) for a ladder.
It also has a great long built in house which hammies like for nesting as it's big enough for a nest, hoard and litter tray and has two entrances. You can fill the base with substrate level to the top of the long house and then just channel out the substrate near the door for an entrance. Putting some substrate inside the house so it's not bare cage base underneath. Plus it comes with two extra houses - loads of space for a robo.
https://www.zooplus.co.uk/shop/small...r_cages/174238