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?
Right, this is the correct answer. Even with a very tall ladder, the force is relatively small. If the ladder is well anchored, you can demonstrate this by standing at the top, holding on tightly, and pushing on the wall to see how hard you have to push to move the top of the ladder away from the wall a little.
Also worth mentioning that they sell covers/pads for the top of the ladders to prevent denting the drywall, but you can also make them from a piece of foam pool noodle.
Heh, fair enough, though if that's enough to make you feel backwards, you don't have enough angle on your ladder.