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?
Get a stabilizer bar for your extension ladder. It makes them much more stable and all the ones I've seen had padded feet on the ends which will also keep you from scratching up the wall.