Here is the full list of all 21 The Watchmaker achievements.
Thomas finds Alexander
You found Aurelius
You found Alice's ghost
You have seen all the memories of Alice
You found Alice's body
You eliminated 1 enemy
You eliminated 10 enemies
You eliminated 20 enemies
You have found 1 watch
You have found 5 watches
You have found 10 watches
You have found 15 watches
You have found 20 watches
You have found all the watches
You found the first Journal
You found all the journals
You found 1 Letter
You found all the letters
Master and Lord of the Clock Tower
The light at the end of the tunnel
You solved the first puzzle