My cat jumped through the second story window of our fort and I fixed it with zip ties and a sheet
Last Saturday I spent about 4 hours building a two story fort in my living room using maybe 14 Amazon boxes, and it had this cool window cut into the upper level. My cat saw a bird outside and launched himself right through that window opening at full speed. The whole top section caved in on him and he came out covered in packing tape like a mummy. I grabbed some zip ties and a old bedsheet from the closet and basically made a little safety net across the window opening. Took me another 45 minutes to rebuild the top floor but the net actually looks kinda cool now. Has anyone else had to cat proof their fort or is it just me dealing with a tiny furry wrecking ball?
That window was already a weak point in the design, so the cat just found the flaw before you did. A sheet held up by zip ties will sag the second he hits it again, so swap in some cardboard taped across the opening with a small cat sized gap if he still needs to peek out. My old roommate's cat did the same thing to a box castle and we ended up just cutting the window higher than he could reach.