Although it may seem counterintuitive, one good way to build a high armor class in Dungeons and Dragons is to forego armor altogether. There are a number of options in the core rules that let you play an unarmored tank. The monk class offers one way. Magic spells offer another. Magic items help too. And have I got a prestige class for you!
The Duelist is designed for characters who disdain armor but want to be secure in battle anyway. It gives an AC bonus equal to your intelligence bonus or your Duelist level, whichever is lower. It gives AC bonuses when provoking attacks of opportunity (AoOs) while moving. And best of all, it gives a +1 per duelist level when fighting defensively -- although that ability does not kick in until 7th level.
Outside of magic items and spells, a duelist strategy depends on having a high dexterity and intelligence. In addition, through monk levels or a Monk's Belt, you'll be able to add your wisdom bonus to your AC, too. So clearly, the maximal execution of this strat depends on high ability scores.
So if you are playing with a low power or standard game, one with, say, a 25-point build for PCs, then this is NOT a strategy for you. A character with, say, 14s in dex, int, and wis, is not getting a very big boost. A high-power game, with a 32-point build, could easily allow a PC with two 16s and a 15. That makes a difference. The same holds true for PCs created by dice rolls: if you roll well, this is an option for you.
But how to get to duelist? There are fast builds and slow builds. A fast build is made with fighter-type classes only, and fighter 6 is not a bad way to go. Ranger is not that great because combat mastery offers only two-weapon fighting (less useful for a duelist because a duelist cannot use a weapon in his off hand) or archery feats (and a duelist is designed for melee, not ranged attacks). A raging barbarian takes an AC penalty, so that doesn't work. A paladin is an option, but really does not offer much in the way of synergies (except that Bless Weapon works very sweetly with rapiers).
The problem with the fighter build for an aspiring duelist tank is that there are not many good options for boosting AC through regular or bonus feats. The fighter/duelist should focus on offense, with weapon specialization, probably in rapier, for a start.
Another strike against the fighter/duelist is that the required skills -- tumble and perform -- must be bought cross class.
Here's one nifty trick -- albeit not one that helps AC all that much -- for a fighter/duelist. A duelist only accrues all his bonuses when wearing no armor and using a one-handed piercing melee weapon. Well, the best one-handed melee weapon is probably the rapier, especially considering that it takes advantage of the duelist's required weapon finesse feat. But if you are willing to use a non-finesse-able one handed piercing melee weapon, what's the best one? Surprise! It is the lance, which is a one-handed weapon when used mounted! Now you have a use for all those mounted feats: Mounted Combat, Spirited Charge, Trample, Weapon Focus and Specialization in lance, the works! Remember to take Combat Reflexes to make best use of your reach!
You might also be tempted by the Rogue 3/Fighter4 option. You still get weapon specialization and three bonus feats, in addition to +2d6 sneak attacks and evasion, PLUS taking tumble and perform as class skills. But once again, this isn't doing much for your AC.
But what about the monk? Using a Monk's Belt already gives you the lion's share of the benefit you seek -- being able to apply your Wis bonus to your AC. So taking just a couple levels of monk hardly seems worth it to me. But if you go all the way to level 8 monk, the benefits start to make more sense. You get a +1 AC bonus on top of your Wis bonus (make that +2 with the Monk's Belt). You can make flurry of blows attacks with your one-handed piercing Siangham, which, as a light weapon, also benefits from the required weapon finesse (as do your other unarmed attacks). You can use your other hand (or feet or knees or elbows, etc) to make the other unarmed attacks in your flurry sequence. Evasion and combat reflexes offer good synergies, and you get tumble and perform as class skills. It's not a bad route to take.
But still, my mind keep turning to the sweet sweet spells that can boost your AC. Spells like Mage Armor, Magic Circle, and Magic Vestment. Also spells like Shield, and non-AC defensive spells like Sanctuary, Mirror Image, and Blink. With a slew of those in place, your survivability takes a big turn for the better. But how to do it?
One path is the Mystic Theurge. Wizard3/Cleric3 gets you started, then take MT levels until you get your requisite +6 BAB. That gives you access to all the spells mentioned above, and more. The problem here is extremely delayed gratification: You don't get that +6 BAB until you are Wizard3/Cleric3/MT6. So your first duelist level comes at level 13, and you don't get your precious elaborate parry until 19th level! Why even bother?
A better way might be through straight cleric levels. A cleric 8 gets great long duration AC boosters like Magic Circle and Magic Vestment -- and has a +6 BAB. If you take the Magic domain, you can use scrolls and wands like a 4th level wizard. That means you can stock up on wands of Mage Armor and Shield. Take Plant domain for your other domain for access to Barkskin. And with your 4th level cleric spells, you get access to great buffs like Divine Power.
The cleric/monk or cleric/rogue multiclass options are tempting, but the big payoff level for cleric is fifth, and fifth level gives you no increase to your BAB, so that's just another delay to your taking the duelist class. If you don't mind the delay, monk5/cleric5/duelist10 is a good 20-level build. Or rogue5/cleric5/duelist10. I think I prefer the monk/cleric over the rogue/cleric because you can deliver touch spells with an unarmed attack. Moreover, if your DM thinks that a Monk's Belt should NOT let you apply your Wis bonus to AC, then your only choice may be to go with a monk multiclass -- in which case the Monk/Cleric with Magic and Plant domains is the best, I think.
But for my exemplar, I think I'll take a halfling cleric8/duelist with magic and plant domains. At higher levels, the Bead of Karma is supersweet, seeing as caster level 12 gives a +5 when casting barkskin and a +3 when casting magic vestment. With a lesser rod of extend, you get a four hour duration on that barkskin and on magic circle.
Let's do the math, sampling at levels 5, 10, and 15.
Level five (9,000 gp): Halfling Cleric5 with 18 Dex (+2 from race), 17 Wis (raised by 1 at 4th), 14 Int. Feats: Dodge, Mobility. Spells: Magic Vestment x2, Barkskin. Gear: chain shirt, heavy shield, +1 ring protection, +1 AC Ioun Stone. (Might as well be armored up before you take your first duelist level.) Armor Class: 27 (28 dodging).
Level ten (49,000 gp): Halfling Cleric8/Duelist2 with 20 Dex (+2 gloves), 20 Wis (another +1 at 8th, +2 amulet), 14 Int. Feats: Dodge, Mobility, Weapon Finesse, Combat Expertise. Spells: Magic Vestment, Magic Circle vs Evil, Greater Magic Weapon, Barkskin, Mage Armor, possible Shield. Gear: Gloves of Dex +2, Amulet of Wis +2, Monk's Belt, Strand of Prayer Beads sans Smiting, Lesser Extend Rod, +1 AC Ioun Stone, +1 defending rapier, Wand of Mage Armor, Wand of Shield. Armor Class: 39 (46 dodging vs evil with Shield up).
Level fifteen (200,000 gp): Halfling Cleric8/Duelist7 with 24 Dex (+6 gloves), 25 Wis (another +1 at 12th, +6 amulet), 20 Int (+6 headband). Feats: Dodge, Mobility, Weapon Finesse, Combat Expertise, Spring Attack, Weapon Focus. Spells: Magic Vestment, Magic Circle vs Evil, Greater Magic Weapon, Barkskin, possible Shield. Gear: Gloves of Dex +6, Amulet of Wis +6, Headband of Int +6, Monk's Belt, Strand of Prayer Beads sans Smiting, Lesser Extend Rod, +1 AC Ioun Stone, +1 Defending Rapier, Bracers of Armor +7, Wand of Shield.Armor Class: 49 (56 dodging vs evil with Shield up).
Add in +5 for combat expertise and +10 for fighting defensively (thanks to elaborate parry), and we're looking at AC71 at 15th level. Not too shabby.
Note that the above math includes a trick. A +1 defending rapier can add +1 to AC, because the defending ability lets you apply the weapon's enhancement bonus to AC. But if you cast Greater Magic Weapon as a 12th level caster on the same weapon, the rapier's enhancement bonus becomes +3. I assume you can apply all three of that to AC.
A final note on the underlying philosophy of building your armor class. This wraps up my series on getting the most of your AC. Since we're topping out at AC71, I think this is a good place to stop! But you have to understand that your AC strategy is a magic item strategy. Since magic items are so key to building your AC, spending your money wisely is vital. Think about how much a new +1 will cost, and choose the least costly option.
For example, enchant your armor and shield first, because a +1 enhancement bonus to armor and shield costs only 1,000 gp. But buy a +1 ring of protection before you upgrade your armor to +2, because the ring costs only 2,000 gp, while upgrading your armor costs another 3,000 gp. Fill each slot and choose wisely. For example, an Ioun Stone of +1 AC costs 5,000 gp. To upgrade your armor from +2 to +3 costs 5,000 as well, so buy the stone, upgrade your armor to +3, and upgrade your shield to +3 all at the same time. But wait to upgrade your Ring of Protection from +1 to +2 -- that upgrade costs an extra 6,000 gp, so do those other upgrades first. Of course, buy the amulet of natural armor +1 before you do any of those other upgrades.
Of course, if you are playing a balanced PC rather than a pure tank, you might want to spend 2,000 gp on a +1 weapon before you buy the 5,000 gp Ioun Stone!
Clever use of gold resources is the great strength of the cleric/duelist build above. By using Magic Circle to get a deflection bonus, you don't need a ring. By casting Barkskin, you do not need an amulet of natural armor (which is great, because you need your amulet slot to boost your Wisdom). By casting Mage Armor, you save yourself the need to buy Bracers of Armor -- until the highest levels, when affordable bracers can beat the +4 bestowed by Mage Armor. So since you do not have to buy rings of protection, magic armor, amulets of natural armor, etc, you can spend your cash on stat boosters, a bead of karma, monk's belt, etc.
This is part of an extended series on Survivability in Dungeons and Dragons.