I started out with a quick perspective view of the full design: ladder, platform, winch, top-pully and rope. The hardest part was how to attach the platform to the ladder so it (a) moved smoothly and (b) stayed on the ladder.
I thought about two options:
Option 1: Wheels on the inside of the ladder:
Option 2: Wheels on the outside of the ladder:
For options 1 and 2 I would need to use flanged wheels to keep the platform on the ladder.
I also considered using roller-blade wheels in a 3rd option: