How to Make Turrets Stronger
Starting with outer turrets..
Fortification is a passive on all turrets, which blocks 30 damage for the next 7 minutes. This is great and all, except it's a little too weak. Turrets are being kicked down at the 5 minute mark, so make it something like 50-60 damage. That would really give it survivability.
You should also add a new passive once Fortification has worn off, lasting the rest of the game. It should give all tier one turrets + 100 AD per turret shot, resetting after it switches targets. Tier two turrets should also get + 20 AD per second.
Just do something Rito.