Ladders can be useful in a gerbil cage, and fun too, but they can also be dangerous, especially for juvenile gerbils. They can fall off them with surprising ease, or ladders and stairs can catch one of their legs, breaking it during a fall.
It can be unlucky to walk under ladders - or up them, if you're a gerbil!
Rather than ladders, It’s best to use access ramps or tubes in an enclosure. Secure them properly to present your pets from falling. Bricks or other stable, heavy objects are a good idea, as these can’t slip or fall.