Despite being advertised as a tanky Cat, Fortressy is not tanky at all, even with hypermax. Adding to that, its cost is 20¢ more than Tank Cat, whom is a much better, more ideal meatshield in every situation, no matter what part of the game you're in. The most this Cat can help you with is being a 4 Crown Stories of Legend meatshield; with the CatcombosBiohazard and Bony Bone supporting it, it can be spammed relatively quickly, despite its low movement speed, not different from how Cameraman Cat is used throughout similar stages.
Despite this, the previously mentioned Cats are a much better investment of XP; so invest in them rather than Fortressy. Overall, the most Fortressy can do to help your Cat Army is increasing your User Rank and, although not recommended, being a meatshield in Stories of Legend.