Using two tower shields is a very bad idea, simply because using them for cover works for Arrows and little else. Targeted effects, spread effects, and burst effects that cover adjacent squares will still affect you. Unfortunately, the biggest threats (usually) aren't Arrows. You'll also leave yourself wide open for anyone to hurt you because you'll have zero offense as well.
My suggestion is Dwarf Ranger. Pick up the Witchguard Archetype. This will make you very capable of defending and aiding your fellow spellcaster party members by being able to aid in AC and concentration checks. Did I also mention you get access to Witch Patron spells as bonus spells to your list?
You'll also get access to the Bodyguard and In Harm's Way feats for free, further helping the "protect people" concept. To make good use of this, however, you'll want some decent Dexterity, and the Combat Reflexes feat. It's actually a smart idea anyway, since this gives you more AC and other goodies as a result.
Take the Weapon and Shield Combat Style. Your feat choices from this is in the following order: Two-Weapon Fighting, Shield Master, Bashing Finish, Shield Focus, Greater Shield Focus.
Assuming a 20 point buy, you should have these stats:
Strength 16
Dexterity 16
Constitution 12 (10 + 2)
Intelligence 10
Wisdom 14 (12 + 2)
Charisma 5 (7 - 2)
You can dump your Intelligence down for more Constitution and Wisdom if you want; you do get 6 or 7 skill ranks per level, so it's certainly something to consider if you don't want to also build to be a skill monkey. Just invest in basic skills like Knowledge (Nature), Survival, and Perception. The rest you can divvy up however you like.
For your other feats, here's your basic rundown (* = bonus feat):
1. Improved Shield Bash
2. Two-Weapon Fighting*
3. Double Slice, Bodyguard*
5. Combat Reflexes
6. Shield Master*
7. In Harm's Way*, (Improved Two-Weapon Fighting)?
9. ???
10. Bashing Finish
11-20. Anything
You'll be focusing on Two-Weapon Fighting, but you can be fine just using one shield to attack with at 1st level. With a 16 Strength, 1D6+3 damage while having approximately 19 AC (4 armor + 3 dex + 2 shield) is pretty good. The killer is having only 20 Move Speed, but wearing heavier armor means you won't be encumbered by its weight, so you may want to consider taking a feat for Heavy Armor Proficiency and just walk around in heavier armor. (Or take a level dip in a class like Fighter for the proficiency, and a bonus feat to use on combat feats you may want. Many options here, though I wouldn't recommend this until at least character level 7.)
The best thing for you to do is stand near your friendly neighborhood caster and give them the buffs they need (while they give you the buffs you'll need). If they're focused on, you have your Witchguard abilities, as well as Bodyguard and In Harm's Way to save their skin, and with 16 Dexterity, that's a lot of attacks you can protect them from, especially from 1st level.
Your level up stats can be used to put points into Strength and shore up your offensive power, though you may want to put at least 1 point into Dexterity for access to Improved Two-Weapon Fighting by 7th level. Of course, if you're focused on defense more than anything, you can delay Improved Two-Weapon Fighting as long as you want and take it when you fulfill its requirements.
Favored Class Bonus can go into either Skills or HP, whichever you think you need more of. The former is good for being relevant outside of combat, the latter is good for making sure you survive combat. Both are good to have, so consider what you already have and factor that into your choice.
Isn't there a new dwarven fighting shield or something in one of the newer books too?
And these are based off of an obscure martial arts weapon too. I guess a monk shield would have been too much to ask though.
