You raised my curiosity enough to look up "feral cat" I learned something new! I used to think feral meant some type of wild cat that looked like a weasel or ferret...Duh!
Anyway, it sounds like you have quite a project on your hands, but it also sounds rewarding if you can get it done.
If I make an assumption, the distance from your window to the ground is about 10 feet? If so, could you use a length of corrugated plastic drainage pipe that is about 12 or 16 inches in diameter. This is the type of pipe that they place under driveways, etc for drainage. It is very light and the corrugations would allow your cats to get traction to climb up. (I assume they have their claws) If you use about a 12 foot piece, the slope may be gentle enough that they would have no problems going up or down. The problem will be in making some type of transition from the pipe to the window. You would also have to drill holes in the pipe to allow ventilation and light to get in. If your cats are like ours, they don't like to go into something where they can't see the other end.
Hope this helps, you'll probably get other ideas.