Although they seem simple, staircases are one of the most time-consuming projects. On top of the ordinances, ergonomic constraints, and state laws that determine what is possible and legal for a staircase, you'll typically get halfway through a staircase when the customer takes a peak and decides they'd rather have horizontal runners instead of vertical pickets.
The staircase above is a solution for a customer with a very old building, built on uneven ground with the walls leaning outwards, and only a small alleyway to build within.
The simple solution would be a straight staircase, but the alley was not long enough and at the overall height of the stairs, code requires at least one platform to break up the trip. The criss-crossing and double back are necessary for avoiding existing doors and allowing the legal rise and run of the treads to fit within the tiny alley.
This one was fairly difficult, but I still prefer it much more to adding a custom handrail to an already built staircase. Each tread has to be measured, because they're never the same.