For me, Joshua, the biggest problem was that it didn't quite hit the theme as I was defining it -- I was looking for things that went wrong because of c*ck-ups or stupid miscalculations or just because of a ridiculous twist of fate. Although there is a miscalculation in that the shelter isn't up to a cat's paw, it's not stupid, in that he couldn't know for sure until it was tested. I think, too, my definitions carry an inherent comedy in them, even if the outcome isn't comedic, and this was too serious to fit.
However, I'm not sure I'd have shortlisted it even if it had appeared in a different Challenge where I thought it fitted the theme. It isn't the most original of ideas, which doesn't help, but I wonder if there's something lacking, though I'm having a problem putting my finger on it. Perhaps because there's no clue as to why they are all so small? Perhaps because there's no sense of emotion? There's no fear or terror or tiredness or alarm, or anything really -- it's emotionally flat.
Certainly there are some aspects of it that I don't think work -- the use of "tribe" for instance, which has specific overtones which aren't fulfilled here, and naming Frank, when we've no idea who he is nor why we should care about him. I also don't like the use of "structure" and "metal lattice walls" which immediately signalled to me that you were trying to conceal what it actually was they were hiding inside, which meant you were trying for a twist finish to the story -- but then you don't tell us what it is, so it's effectively wasted words. (You can't rely on people reading the piece several times and thinking "D'oh! It's a sieve!" (or whatever) -- you have to spell it out first time round, and at the end if you want a twist ending.)
Actually, now looking at it in detail to my mind you've wasted a good many words eg "holstering his spear" (and spears aren't usually holstered), the "Fate decreed" sentence, "Without warning", "wooden point". None of these are necessary to carry the story forward nor do they give enough flavour or atmosphere to the whole. I imagine the spear is a toothpick, which could have given a real punch to the ending if used, but again it's lost as a plot point if it's not made obvious. By the way, I love "schiltron" but I'm not convinced it's the right term since if there's only one cat they surely don't need to defend simultaneously from several different directions.
Basically, it's a workmanlike story, but for me it lacked a spark which perhaps might have come if you'd pruned out the unnecessary words and given more atmosphere to it and/or more backstory.