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?
Sounds like you have it covered friend. I use two washcloths and tape them on the end of the ladder where it will come in contact with the wall to prevent scuffs. Set up is exactly like the exterior.
Definitely make sure that the ladder has non-stick feet or grab some non-slip material or a doormat with a non slip surface. Ensure to test it before going too high. Always best to have a spotter.