Tower Shields need to offer more protection. At high levels it is relatively easy for a character to have more dexterity then a tower shield will allow them to use and this makes large shields offer equal or even better protection.
In order to make use of a tower shield you have to heavily spec for it and even then it offers very slight increase over large shields. The PRR and AC that tower shields give both need to be buffed.
A little math:
Starting dex of 10 +2 for a Tome +2 for ship buffs and +8 for item buff gives you a dexterity of 22 very easily doable.
A Mountain tower shield gives you damage reduction of 7 and a base AC of 10, while A large Spiritcraft shield gives you a damage reduction of 6 and a base AC of 7. With a dexterity of 22 that means that the Tower shield gives you an AC of 12 while the Large shield gives you an AC of 13!
At the highest level the AC for Tower/Large shields is only 12 and 9 and the dexterity only increases making Large shields the better choice by far.
The base AC for the 5 levels of tower shields are 4/6/8/10/12
I would suggest the tower shields AC base be increased 5/7/11/14/16 and the base damage reduction increased by a straight 2 accross all tower shields.