Why are constructs and all undead immune to crits. For some of them it simply doesn't makie sense.
How so does it not make sense? They do not have vital organs (in the case of constructs) or damage to vitals doesn't inflict any additional injury or pain than damage to any other body part (in the case of undead). Both are animated by magic, they are not given life of their own (except maybe in Ravenloft), and the body is held together by that magic. The body as a whole must be damaged to a point where that magic is no longer capable of animating the body. You take a longsword (or whatever) to a zombie, it causes roughly the same amount of damage (1D8) wherever your blow lands. They do not feel pain, sense injury or anything like that, so slashing at its torso is no different than running your sword through its face! Their animation is not tied to the function and integrity of internal organs, in fact, they have no organs (constructs) or what they do have has long stopped functioning (undead).
But skeletons, golems, zombies etc... these guys should be subject to crits. Afterall you can target bits of their bodies which are weaker and which would have a more dramatic effect on their ability to fight.
Again, these things are animated by a magical force. They do not have 'weak' areas. How are you going to critical a skeleton? It has no flesh, organs and is entirely bone. Slashing and stabbing weapons do less damage against them because most of the damage from these weapons is from slicing open veins, arteries and organs--all of which are missing in skeletons. That aside, you
cannot target bits of their bodies, no Called Shots. You attack undead or constructs with a shortsword, you inflict 1D6 damage (plus modifiers, etc.)...period. Its up to the DM to describe the effects of that damage. If that target was a zombie who had only 3HP left and you just did 6HP damage, then the DM could say something like,
"Your last blow takes the head from its shoulders and it collapses into a mass of quivering flesh as the last of the magic animating it leaves the remains." Or maybe the blow cleaved it in half. Whatever. For most undead and constructs, 0HP = destroyed. And that's all you can do. Whittle away at it until you have done enough damage to the body overall that it can no longer be animated.
Take out a zombie's knee. OK it's not dead, and it's not hurt a great deal more than if you put your sword through it's chest... but it is suddenly crwaling around rather than walking. Or it's trying desperately to balance on a single leg while it fights. As the Hit Point system is an abstraction this has a dramatic effect on your ability to put in the 'killer blow' that would take it out fully. As it crawls toward you it practically opens itself up for a CDG.
Take out the elbow, and suddenly it's not working so well at hitting you. It's undead so it's hardly going to adapt quickly enough to hit you with the stump. And the result is that you can get in a fraction closer on that side and again are more likely to get in the important strike.
Skeletons, ghouls, zombies, constructs - they all have these weaknesses. OK so they're not the same as 'normal' crits where you going for vital points, but they should still be allowed.
And how do you take out that zombie's knee without the DM allowing Called Shots? Who says the zombie is crawling? Who says it
has to balance? It is an animated object; that means a magical force is making it move, not that it relies on muscles or any physical features to keep it upright.
How would a crawling zombie be any more or less vulnerable to a CDG? It has no vitals, cutting off or destroying organs (again impossible without house ruling Called Shots) doesn't inflict any more damage than any other wound it receives. If its crawling (for whatever reason), that just means its not attacking, not that its any more vulnerable than when it is upright. Same thing with the elbow: How are you going to 'take it out' without the DM allowing Called Shots? How is that going to make it less dangerous? It still has one other arm to pummel you with, two other legs to kick you with, even a mouth to bite you with!
How about a system to make it harder to crit these creatures.
They would have to be vulnerable to critical hits before you can make it harder to crit them.
I can see your point of view, but working within the mechanics of D&D 3.X, you've not given any reason to justify making constructs or undead subject to critical hits. I don't think this should have anything to do with 'game balance' or anything like that. The entire CR system is flawed anyway. If you think the undead or construct in question is tougher than its CR indicates just because of its lack of vulnerability to criticals, then raise the CR. Then again, if you want them to be vulnerable to crits, its your game, house rule it that some of them are. Then just be ready to reduce their CR when the fighters with Improved Critical and/or keen weapons and rogues with high sneak attack damage make short work of your monsters.
Of course, you could rule that a critical hit provides a different effect instead of extra damage. In regards to what Hero4hire stated, maybe a 'critical' against a vampire would put that stake (or arrow, or other piece of wood) through its heart and maybe just immobilize it or destroy its body and turn it to mist forcing it to retreat to its coffin? Maybe a 'critical' against a zombie or skeleton knocks it down or knocks it 5' or 10' away from you? Maybe a 'critical' against a stone golem is a hit that strikes hard enough to give it a penalty to its next attack or maybe strikes it so hard that the blow ignores a portion of its DR? There are all kinds of possibilities that you could put into place without making them subject to critical hit damage (and thus SA damage).
But that zombie is significantly more affected by a blow that decapitates him than a similar blow to the body. THe blow to the body just damages him, the decapitation prevents him from seeing you (OK assumes that they still 'see' using their eyes and not some 'life sense', but that's my assumption). Given that the hit point system is such an abstraction you have just dramatically improved your position in the battle, making it more likely that you would slay the zombie. An accurate way of reflecting that is in the hit point system. As d20 does not have an official 'called shot' system (that I know of), you can't have a decapitating blow - otherwise you could impose normal penalties for your opponent being unable to see you. However it would be a plausible means of taking down a zombie effectively, so the only way to represent it is the hit point system - by allowing crits.
Again, how do you figure a zombie is anymore affected by a blow to the head than anywhere else? Decapitation (except perhaps in your campaign) wouldn't prevent it from 'seeing' you. Zombies don't see. They are mindless and all the organs in their body are dead. Even if their eyes worked, they are mindless and would not have the brainpower to process the info the eyes are transmitting to the (dead) brain. How undead sense things is entirely up to the DM, but mindless undead are mindless, they do not think/act on their own but by the command of the person that animated them. Maybe there is an unspoken command to attack any living creatures while not obeying its animator's command?
You keep mentioning, twice now, that the HP system is an abstraction that makes it easier to slay zombies. Reversing that logic then, why wouldn't the zombie's 1D8HP slam attack be able to put its fist through
your face, if you could do the same to it with your own 1D8HP attack? If you can do the same to it, what's to stop it from doing the same to you? Rules? How if HP are an abstraction? It would have the same chance of tearing off your head or ripping one of your arms off according to your logic.
In a system where Hit Points are an abstraction then a crit represents the fact that a single blow has a significant detrimental impact on the ability of a creature to survive the fight. Any creature that can be incapacitated or impeded (relatively to it's 'normal' state) with a single blow should by subject to crits. By severing the leg of the zombie, skeleton, whatever you can do that.
A single blow can be enough to incapacitate anyone or anything if they have few enough HP. That alone doesn't justify vulnerability to crits. And unless the DM has a house rule, a critical hit that inflicts 30HP is no different than a regular hit that inflicts 10HP when laid on a target with 100HP. Neither blow impedes or incapacitates that person. They are not moving any slower or landing weaker blows. And no DM would tell his 100HP player that he just lost his leg (or arm) from a 30HP blow, so you can't really justify hacking off limbs or whatever on undead if you're not willing to do that to the living (including PCs).
