You do want, of course, your Hero to turn around so it actually faces the enemy it's shooting at, right?
If you don't, set the movement speed to 0...
Assuming you do:
Find the ability "Root (Ancient Protector)", and set it up like this:
- Data - Rooted turning: true
- Data - Rooted Weapons: attack 1 (optionally both)
Find and copy&paste the ability "Spell Book".
Set it up:
- Data - Min spells: 1
- Data - Max spells: 1
- Data - Spell list: the Root ability you edited previously
Give the Spell Book to all towers based on units.
Disable the book for all players:
Trigger:
- Events
- Map initialization
- Conditions
- Actions
- Player Group - Pick every player in (All players) and do (Actions)
- Loop - Actions
- Player - Disable Spell Book (Allow Turning) for (Picked player)