Understand the two separate dangers
Window-proofing a flat has to solve two different problems. The first is falling: cats chase insects, misjudge sills, lean on weak screens and slip from heights that cause serious injury, and living several floors up turns a small mistake into an emergency. The second is entrapment: a cat who tries to squeeze through a narrow gap, especially the V-shaped opening of a tilted window, can become wedged by the hips or neck and injure herself badly while struggling, sometimes within a short time and without anyone hearing her.
Fit screens that are actually built for cats
Choose screens or mesh panels designed for pet use, fixed firmly into or across the frame so a cat leaning her full weight, kneading or launching herself at a passing bird cannot push them out. Lightweight insect mesh is made to stop flies, not animals, and a determined cat can claw through it or pop a loosely fitted panel out of its runners. Check how the product fixes to your frame type, measure carefully, and test the finished screen with firm pressure from inside at several points before you trust it.
In rented flats, ask the landlord before drilling into frames and look at the clamp-fit and tension-fit systems made for rentals if fixings are refused. A removable but genuinely sturdy screen that you are allowed to install beats a permanent one you never put up. Whichever system you choose, inspect it regularly for claw damage, loosened fixings and warping, because a screen is only protective while it is intact.
Control how far every window can open
For windows without full screens, fit restrictors or locks that limit the opening to a gap her head cannot pass through, and remember that a cat's body follows where her head and shoulders fit, so judge the gap by her head, not by what looks small to you. Apply the same rule to every reachable window, not just the one she currently favours, because cats redistribute their habits the moment a new sunny sill appears.
Tilt-and-turn windows deserve special treatment. Either keep them firmly closed whenever she has access to the room, or fit the protective grilles made to cover the side gaps of the tilted position, so the inviting V shape is blocked completely. Never leave a tilted window unguarded in a room she can enter, even briefly, and make that the single strictest window rule in the flat.
Work through the flat room by room
Audit the whole flat as a cat experiences it: windows above sofas, beds and shelves she can launch from, bathroom windows left open for steam, skylights, balcony doors treated in the same pass, and any window a visitor might open without thinking. Check what is below and beside each window inside, because a bookcase beside an unscreened window converts a safe window into a reachable one.
Make the rules survive other people
Window accidents in flats usually involve a second person: a guest who opens a window fully on a hot night, a cleaner who tilts one for air, a partner who assumes the screen is decorative. Walk everyone who stays in the flat through which windows never open, which only open behind screens, and why the tilt position is banned, and put a small reminder near any window that causes confusion. Rules that live only in your head leave the flat with you.
That is where keeping the flat's safety details written down and shared earns its keep, and KinsLe's Family Sharing does that job for your household, keeping your cat's profile and care information, including the window rules for each room, in front of every family member who helps look after her.