Here is the full list of all 61 Hidden Cats in Rome achievements. It takes around 1-2 hours to unlock all of the achievements on Windows.
Finish with no Hints Advanced
Nerd Arena Completed
You Found 800 Cats
Basilica Completed
You Found 700 Cats
Fontana Completed
You Found 600 Cats
Pastificio Completed
Gelateria Completed
You Found 500 Cats
Sanctuary Completed
You Found 400 Cats
Osteria Completed
Found All Cats Normal
Finish with no Hints Normal
You Found 300 Cats
You Found 250 Cats
Found All Cats Advanced
Fontana Discovered
Castle of the Holy Angel Discovered
You Found 200 Cats
Roman Forum Discovered
You Found The Legionary
Mausoleum of Augustus Discovered
Spanish Steps Discovered
Pantheon Discovered
You Found Tedesco
You Found 150 Cats
You Found The Senator
You Found Giulio
You Found Cesar
You Found Maminha
Colosseum Discovered
You Found The Guardione
You Found 110 Cats
You Found 100 Cats
You Found Mauro
You Found 95 Cats
You Found 90 Cats
You Found Chiquinha
You Found 85 Cats
You Found 80 Cats
You Found The Holly One
You Found 75 Cats
You Found 70 Cats
You Found 65 Cats
You Found 60 Cats
You Found Leone
You Found 55 Cats
You Found 50 Cats
You Found 45 Cats
You Found 40 Cats
You Found 35 Cats
You Found 30 Cats
You Found 25 Cats
You Found Chierichetto
You Found 20 Cats
You Found 15 Cats
You Found 10 Cats
You Found 5 Cats
Vatican Discovered