The pixel art was excellent and I loved the broom animation. Very detailed! I also loved seeing the cats in the upgrade armor, especially <null> cat. Would be really interesting if each cat had special attack properties! The names and visuals of the cats just felt so cool, like if it was an XCOM campaign style game, I'd grow attached to each one in the party. Of course I'd make <null> my overpowered party leader and feel really cool.
I think one thing that might help is keeping the upgrades even after the cat runs away for food? I felt a bit disappointed when I spent 25 mana and lost everything from not clicking the fish in time.
But overall it was really nicely done and I had fun! Definitely didn't expect a tower defense from the title, but it was a pleasant surprise!