I think the offensive power of tower is okay...
However, the defense they have is laughable. They go down in seconds, and "poking" them down is extremely easy to do. This is especially true for Inhibitor and the Nexus turrets, which are supposed to be the last line of defense.
I believe that they should get defensive buffs, both to their base stats, and as the game progresses. Something like this would do the trick I believe:
'1. Outer Turrets
Health: 2000
Armor: 100
MR: 100
HP5: 0
Items: Penetrating Bullets, Reinforced Armor, Warden's Eye, Fortification
No changes here, I think the outer turrets are spot on.
'2. Inner Turrets
Health: 2300
Armor: 100 (+5 for every minute after 20, maxing out at 150 at 30 minutes) (from flat 100)
MR: 100 (+5 for every minute after 20, maxing out at 150 at 30 minutes) (from flat 100)
HP5: 0
Items: Penetrating Bullets, Reinforced Armor, Warden's Eye, Fortification, Vanguard (only top and bottom)
'3. Inhibitor Turrets
Health: 2500 (from 2000... Really, they are squishier than Inner ones?)
Armor: 100 (+5 for every minute after 20, maxing out at 200 at 40 minutes) (from flat 100)
MR: 100 (+5 for every minute after 20, maxing out at 200 at 40 minutes) (from flat 100)
HP5: 5
Items: Lightning Rod, Reiforced Armor, Warden's Eye, Maintenance
New turret item: Maintenance - increases turret armor, MR and Health Regen by 4 for every allied champion nearby (1100 range).
'4. Nexus Turrets
Health: 3000 (from 2500)
Armor: 100 (+5 for every minute after 20, maxing out at 200 at 40 minutes) (from flat 100)
MR: 100 (+5 for every minute after 20, maxing out at 200 at 40 minutes) (from flat 100)
HP5: 5
Items: Lightning Rod, Reinforced Armor, Warden's Eye, Maintenance
This wouldn't make tower dives any easier, but it would make breaking the turrets harder to do - especially if there are enemy champions nearby