I wanted to make a small, portable PrC to turn any martial-bent character into a strong tank with enough durability and defensive abilities to contribute to a party and, still have enough levels left over to work on being flexible and adaptable outside the niche of the PrC.
Sorry to see you're getting no love here. I think you're right about there not being a really good tank/defender/bodyguard type Prestige Class or base class in 3.5. That Defiant Defender looks all right, but I have a couple of problems with it: no options, and two abilities that only help undead with no alternate abilities granted to the living. So I took what I felt was most important (forbidding charges and shielding allies) and made my own 5 level Prestige Class. They get 5 defensive tricks, one per level, but they get to choose which ones they want between 12 options. I also loosened the requirements a bit (why require both Improved Trip AND Improved Disarm if there isn't really any good synergy between those two and any of the abilities?)
Requirements: Base attack bonus +5, Combat Expertise, Combat Reflexes
Skills: 2 + int modifier per level
Class Skills: Intimidate, Spot, Listen, Sense Motive.
A Stalwart Guardian gains proficiency with tower shields if he had proficiency with all other shields.
A Stalwart Guardian gains a defensive trick every level. Some tricks have a minimum level or ability requirement, and most grant increasing benefits with every added level of Stalwart Guardian.
Bodyguard: You may occupy an allyís space without penalty. While in the same space as an ally, you may absorb half the damage of attacks made against them, up to a number of attacks per round equal to your dexterity modifier +1. While under a mind-affecting spell or effect, you gain an immediate reroll on your saving throw if you are ordered to attack an ally, with a bonus equal to your Stalwart Guardian levels.
Ambush Sense: You may always act during a surprise round, and your Stalwart Guardian levels count as two rogue levels when determining if you can be sneak attacked (if you have 2 rogue levels and 4 Stalwart Guardian levels, you count as a 10th level rogue and can only be sneak attacked by a rogue of 14th level or higher). You can still be flanked and are still flat-footed in a surprise round if you haven't acted yet. This does not count as Uncanny Dodge or Improved Uncanny Dodge.
Tactical Instincts: You gain a bonus to your initiative and extra attacks of opportunity equal to your intelligence bonus, but no more than your levels of Stalwart Guardian.
Stance of the Dwarf Lord: Any round in which you donít move, you gain a circumstance bonus of +2 per level of Stalwart Guardian to resist being bull-rushed, overrun, or tripped and to your Armor Class vs charge attacks. Your movement speed is not reduced by wearing medium or heavy armor or carrying loads below your maximum heavy load.
Hedge of Spears: You gain a bonus to attack and damage equal to your Stalwart Guardian level to attacks of opportunity, which doubles for attacks of opportunity prompted by moving through your threatened squares.
Armorskin: You gain DR/- equal to the armor bonus of any armor you wear (not including enhancements), but no greater than your Stalwart Guardian levels. This stacks with other sources of DR/-, such as from an adamantine breastplate, the shield symbiote ability, or levels of barbarian.
Shield Symbiote: You gain DR/- equal to the shield bonus of any shield you use (not including enhancements), but no greater than your Stalwart Guardian levels. This stacks with other sources of DR/-, such as from an adamantine breastplate, the armorskin ability, or levels of barbarian.
Wall of Steel: When you make use of your Combat Expertise feat, you add an extra +1 to your AC per point of penalty to your attack roll, up to the number of levels of Stalwart Guardian you have, and any allies who are attacked by an opponent you threaten gain the same bonus. You also add the same bonus to all saves.
Deft Parry: You may sacrifice an attack of opportunity to intercept incoming attacks from opponents you threaten. You make an attack roll at your highest base attack bonus plus any modifiers that would apply to a disarm attempt. If your result exceeds your opponentís, their attack automatically misses. If their result exceeds yours, they must still overcome your AC to hit you. If you exceed their roll by 15 or more, they are considered to have made a critical fumble and provoke attacks of opportunity. The number of times you may use Deft Parry in a round may not exceed your attacks of opportunity or the number of levels of Stalwart Guardian you have.
None Shall Pass: Enemies who try to move through your threatened areas treat it as difficult terrain, and if tumbling, add your levels of Stalwart Guardian to the DC of their tumble check. When you hit an enemy with an attack of opportunity provoked by moving through your threatened area, you may choose to deal only half damage and prevent them moving out of the square that they were in when they provoked the attack of opportunity.
This is Sparta: You may treat a longspear as a one-handed martial weapon and gain the ability to lunge 5ft when armed with one, increasing your reach to 15 ft for a medium character, while still threatening squares 10 ft away. You also gain the ability to treat a tower shield as a one-handed martial melee weapon dealing damage equal 1d6 + your Stalwart Guardian levels. You must have at least 15 strength to select this ability.
I Move For No Man: In any round in which you donít move, you gain a +2 morale bonus to AC, saves, and opposed rolls, such as disarms, trips, bull-rushes, and grapple checks. This bonus increases by another +2 for each round you donít move, up to the number of levels of Stalwart Guardian you have. The bonus goes away if you are moved by a bull-rush or an enemy who grapples you.
Adroit Defender: Your Stalwart Guardian level is counted as one higher than it actually is for the purposes of determining the benefits of your defensive tricks.
Spurn Deathís Embrace: You do not immediately die when reduced to -10 hitpoints. Once you go below -10 hitpoints, you survive for one round per level of Stalwart Guardian you have, no matter how much damage you receive. You must be at least a 3rd level Stalwart Guardian to choose this trick, and it only functions while you are wearing heavy armor.
Armored Soul: You gain spell resistance equal to your Armor Class. You must be a 5th level Stalwart Guardian to select this trick.
Well, the reason I added the undead abilities, is because undead use a d12 for each HD regardless of class, so I wanted to help give undead who built their character around being a mundane martial character a small hit point bonus over what they would get for doing something else more subtle or magical.
And besides, undead already pay for immunity to mind affecting spells and effects (among other things) in the form of a level adjustment or level loss (necropolitan) so I decided to give them something instead of loyalty which would be useless to them.
I left specific types of weapons, armor and shields out the prerequisites and the rest of the class, because I wanted to diversify the types of characters who might want to play the role the class entails.
Ahh, I see where I misread something before. Loyalty IS a different benefit for the living and the dead. Uncanny Resilience, however, remains strictly for benefitting undead and constructs and I believe it should become a feat instead; it doesn't seem to fit with a defense-oriented class, unless more of the class was geared towards undead martial characters (Tombwarden might be a good name for such a thing).
Anyway, your original stated purpose, I feel, is better accomplished by my Stalwart Guardian (relevant for any martial character who wants to become a tank), but your Defiant Defender fits better with the specific build that you have in mind. You certainly have my permission to use any of the concepts I've laid out, but keep in mind that only you and I have checked it for balance. Obviously, I like my own work better (human nature), but one of the things I like about it is the choices. I suggest reevaluating your Defiant Defender from the point of view of various builds.
Well, it occurs to me now, that the high dexterity needed gain additional attacks of attacks of opportunity, kind of goes against the grain of using heavy armor. So I'm glad I left out including any armor requirements and abilities.
Hmm, I have to admit I just dismissed the idea of tanking without heavy armor as pure cheese, but even my own Stalwart Defender could be viable as a high dex character focusing on attacks of opportunity by selecting the correct tricks (and should do so, since combat reflexes is a requirement for entry). I will drop the armor and shield proficiency requirements. I think I will add a trick that grants extra attacks of opportunity, as your combat intuition (though I will have it based on intelligence, since they must have at least 13 intelligence to get combat expertise). I also made him fairly safe from chargers and gave him a couple of ways to boost his saves and gain SR, but I don't have a trick that would help vs sneak attacks yet. Changes to come.
I still maintain that instead of the immutable order you've laid down for the defiant defender to receive his abilities, you should let him choose as he goes.