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Could either bolt through or screw into the solid battens behind it. I'm confident that a fixing like a coach bolt properly attached would mean that effectively the whole window frame/sill/under-sill cabinet would need to come loose in order for the ladder to fall (and that's bigger than the window!), but what I'm not sure about is what type of bracket/climbing anchor etcto fix to the wood with a coach bolt, if you catch my drift!
In climbing centres what do they use at the top of a top rope route? I'd assume something like that would be best.
Spxtz-
https://m.petzl.com/INT/en/Professional/Anchors/COEUR-BOLT-STEEL
If your rope hangs out the window make sure that whatever part of it is touching the window and under pressure is sheathed and protected so it can't be cut.
skinny
Climbing anchors will be way over-built, the weakpoint will almost certainly be what you are fixing into. Will you be screwing it into the wooden cabinet or do you have access to bolt through it? Will the slings then hang straight out of the window?
I would happily trust my life to 2 x 10mm lag bolts or similar if they were well tightened into solid wood and loaded in shear.