The advent of Forge slots made it so that any weapon with a forge slot was potentially as good as its intrinsic pierce or resist, PLUS the best available pierce/resist forge components. For example, a sword with NO intrinsic pierce was still better than anything up to +45 intrinsic pierce, if the former had a forge slot and the latter didn't. The inflation ("rebalancing") of forge item stats and introduction of new tiers (especially silver ingots) had the overall effect of somewhat depreciating the value of intrinsic pierce/resist.
The proliferation of Warbringer and Retribution gear, elite tiers of same, and legendary alchemies all with various combinations of exceptional pierce/resist and slots, makes gear goal choices quite complicated. Drawing on the wiki's Legendary Alchemy lists, Piercing/Resistance page, and Forge page, I endeavored to assemble a new "Best in Slot" (BiS) list to replace the old one based strictly on raw att/def.
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When I first started this analysis, I thought it was possible to put a silver ingot AND a bz ingot stack on a 2-slot item. Maybe at that time, it was and they closed the loophole since. Either way, the best you can do now on a 2-slot item is silver ingot (45 pierce or resist) and shining earth gem (25 pierce/resist AND 25 earth offense/defense). If elemental offense works the way I think it does, it is basically more pierce, so a shining earth gem is effectively 50 pierce (25 physical + 25 earth). The nuances of specific resist factors muddy that analysis a bit, and the converse (earth resist vs. no pierce) is not a symmetrical equivalence.
In short, the general pattern for optimal offensive slotting is: shining earth first (50 effective pierce), silver second (total 95 effective pierce), dull gem of choice third (125 effective pierce). For defensive slotting, you'd primarily want a silver ingot (45 resist), then a shining earth gem (total 70 physical resist), then dull gem of choice (total 85 physical resist).
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I have said in the past that offense is king in Castle Age, based on the ability to buy bronze pierce ingot stacks, while bronze resist stacks must be alchemied from farmed loot drops or gambled from chests. Silver ingot rarity levels that playing field a little but not completely, so I remain committed to a build strongly unbalanced in favor of pierce. So let's look at goals in a Pierce build first.
Conquest paths offer a lot of pierce goodies - a whole outfit each of pierce and resist, in fact. The Pierce sets are Warbringer and Elite Warbringer. With about a year of fairly dedicated Conquest Medal farming, but little FP use toward Conqueror/Guardian points, I've managed to complete my first-tier Warbringer (WB) set and I'm close to done with the first-tier Retribution (Retr.) set. Every WB item is worth the coins. Only 2 of the first-tier set are forge-able though: the WB Plate and the WB Longblade. And since armor only accepts resist forges, I suppose you could forge something on your WB Plate just in case someone takes a swing at you while you're geared for offense, but I wouldn't invest in that until I had a solid resist build forged to the max. And by that time I might easily have Elite WB Plate acquired anyway.
So let's talk about the Warbringer Longblade. Actually, let's back up a step: If you're just starting out in conquest paths, I'd recommend first snapping up a Duke's Bone Blade ASAP and loading its 2 slots with the best pierce ingots/gems you can get your paws on. This could give as much as 80 pierce (silver ingot + bz stack). Even a bz stack and earth gem will give 60 pierce. This will give you a significantly extended reach in PvP until you unlock WB Longblade.
WB Longblade intrinsically packs 40 pierce, and with a silver ingot in the forge slot it maxes out at 85 pierce. If you're not a millionaire plowing FP into speeding through conquest paths, this will be your PvP weapon for quite a while, so make sure your Attack rune is on it when you forge it too.
The next step up is Aqua Glaive, an epic alchemy from Leviathan of the Grove that offers just 5 more pierce. Leviathans do not seem a worthwhile diversion when you could be investing the stam in monsters that give more dramatic return on investment. The Elite iteration of Warbringer Longblade is a no-brainer when you get there, superseded only by the 3-stage insanity that is the Cronus Astaroth alchemy project weapon. If you're chasing that then you don't need my advice, but let's take a moment to drool over the final Astaroth Blade stats: 140 intrinsic pierce and 2 forge slots for a potential max of 220 pierce.
The natural complement to weapons are gauntlets, because that's your offensive rune-bearing equipment. Sadly, the first-tier WB Gauntlet (20 pierce) has no forge slot, making it worse for its purpose than Knights Bone Gauntlets with no intrinsic pierce, but a forge slot potentiating up to 45 pierce. If you didn't suck at Arena a couple seasons back, perhaps you won a Vanguards Gauntlet: basically a Knight's Bone Gauntlet with no intrinsic pierce, a forge slot, and somewhat better raw att/def stats. The Elite WB Gauntlet, with 30 intrinsic pierce and 1 forge slot (max 75 pierce) is the ultimate pierce glove, short of the legendary Hordeslayer Gauntlet to Deathgauntlet path.
The nifty thing about off-hand equipment is it can take pierce or resist ingots/gems. In the intrinsic-pierce category, the first-tier WB Shield is OK with 20 pierce and no forge slot. The problem with the next step up, the Gorgon Protector with 25 intrinsic and a forge slot for up to 70 pierce max, is you have to own a boatload of other, lackluster project alchemies. I would just hold out for the Elite WB Shield: 30 intrinsic + 1 forge slot = 75 max pierce.
Armor and helmets cannot take pierce ingots/gems, and the alternatives to the WB bits (Leviathans Plate, Sea King's Helm, Helm of Zeus) don't seem worth the effort for the marginal gains they provide over first-tier WB items. And then they're superseded in the end by Elite WB Plate and Elite WB Helm anyway.
There are currently no forge-able boots or amulets, so Warbringer and Retribution are pretty much your "sole" options there. And not only is magic not forge-able (which seems quite logical), I suppose there could be but there are not any magics with intrinsic pierce/resist.
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Retr. Hammer and Elite Retr. Hammer are the only weapons with any resist, and weapons can't take a resist ingot/gem forging, so that's a short topic. Likewise gloves, except that eyebrow-raising Thanatos Gauntlet with 85 intrinsic resist and 2 forge slots. Unless they bent the rules and allow resist ingots/gems to be forged on that specific glove, I would not chase it until just about every other possible resist-build goal has been met.
In the off-hand ("shield") slot, the Retr. Aegis has no forge slot (20 resist), but the Elite version (30 intrinsic resist) does have one, so that's good for up to 75 resist. The next step up and currently best-in-slot defensive off-hand is the Gorgons Heart. Not to be confused with the offensive Gorgon Protector, the Heart packs 80 intrinsic resist plus a forge slot for a max potential 125 resist.
That brings us to the equipment slot where resist should shine brightest: armor. There are a plethora of armor pieces with zero intrinsic resist and one forge slot. Armors with any intrinsic resist all have at least one forge slot. And there is Commanders Bone Plate, with zero intrinsic resist but two forge slots, making it a dark horse early choice for up to 80 resist - better, in fact, than Retr. Plate (20 intrinsic resist and 1 slot for a max of 65), or even Elite Retr. Plate (max 75 resist). And then Cmdr. Bone Plate is the last word in resist armor until the Death Rat alchemy 2-stage project Deathmantle (75 intrinsic + 3 slots = 180 max resist) and 3-stage Thanatos Plate (140 intrinsic + 2 slots = 220 max resist). The one piece where you might expect the Retribution set to be the go-to choice before legendary alchemies, it falls flat.
Where the Retribution sets disappoint in armor, they make it up in helmets. The first-tier Retr. Helm is a solid 40 resist, although it sports no forge slots. I happened to have a nice Warrior's Helm with 0 intrinsic but 1 forge slot, from some Arena season. That, Sea King's Helm or Helm of Zeus could be a goal if 5 more resist means that much to you, but I personally wouldn't plod through the prerequisite alchemy project paths for either of the latter two just for that. I expect there is some uber resist helm on the horizon, but until that's released, the next and best-in-slot resist helm is Elite Retr. Helm with 60 intrinsic resist and not one, but TWO forge slots, for a max possible 140 resist - better by far even than any of the armors short of the very painful legendaries.
As with pierce gear, there are no boot or amulet choices off the conquest path, so simply buy the Retr. Greaves and Amulet and then the Elite versions as you unlock them.