Title is pretty much everything!
I have these high up windows that I'd like to clean and maybe in the future install curtains on. I guestimate the wall being 20ft (6m) tall.
For other rooms in the house I typically use an A-Frame, but it's impossible to find an A-Frame to reach that high where I am.
I plan on draping a towel over the top of the ladder to prevent scratches and scuffs on the wall, as well as getting a rubber mat or something if the floor seems slippery. I am just not sure if drywall would support a ladder? Especially near the frame of the windows. Maybe there's something else I'm not considering as well?
An extension ladder shouldn't damage the wall unless you lose control of it or bang it. Wrapping the end with a cloth or buying rubber caps can help. When you move from side to side, try to lift the top away from the wall so you don't scratch it; this is the most common way you could damage the wall.
They make stabilizers that often come with caps, but they will push you away from the wall; if you have short arms, this might be a concern. They also will require more space from the wall for the base to rest so you aren't vertical.
Always try to have a buddy at the bottom of the ladder to make sure the base doesn't slide out from under you, especially if you have anything other than carpet.
Optimally, you will want to try to have your ladder top rest on the windowsill or lower trim that frames the window, without it touching the glass.