The stairs with alternating treads are a viable option to ladders, retractables, spirals stairs or extensibles.
These stairs occupy little space and therefore they behave a shortening of the general length of the staircase.
Basically, every step is studied for being of support only for one of our feet: along the stair successive treads only for the support of the left foot and treads only for the support of the right foot.
This proposal is apparently a bit uncomfortable, but it's a good solution to reach mezzanine, attics or areas uncrowded and once tried and understood the mechanism of it, get on and off will become very natural.
The DIXI model of Mobirolo is a modern and brilliant solution for smaller spaces. The design of the steps is optimized to allow a surface of adequate support to cross the staircase and reduce the overall dimensions.