No no, I mean, I think after 3000 years, there would be no guardians left in the ruins...That would be boring of course from a gaming perspective, but...No ones taken them out yet?

But that does seem the most logical explanation, unless Necromancers have had dealing in EVERY ruin possible.

I think several of you are right. Most likely the Ayleids made guardians. There is at least one book mentioning it. Perhaps they also "cursed" their homes. It is rather obvious that the Ayleid magic is very endurant. THe amount of power that survives today is a great proof to that. A chapel undercroft doesn't curse itself, someone has to do it. And I can think of at least two major chapel undercrofts that are haunted (Cheydinhals and Chorrols). If someone can curse an undercroft the Ayleids should be able to put a similar spell to their own homes. The rest is a logical process:
1: Guardians raised and the tomb is "cursed".
2: Flesh guardians rot and become skeletal, skeletal guardians wither and become ethereal.
3: Adventurer, tomb raider or other living person enters the ruins.
4: The entering people are killed.
5: The dead are raised by the curse.
6: A few repeats of stade 2-5
7: The ruin is cleared out and looted, but perhaps some is left behind.
8: The ruin eventually occupied or entered by bandits or monsters.
9: Some of the new inhabitants are killed by traps or adenturers.
10: Repeating step 5
11: If the tomb is empty the dead might get confused and start to "reclaim" the treasures, thus leading to them raiding the countryside.
12: Repeating steps 2-11 in eternity or untill the curse is removed or ends.

If the curse can recharge itself then that can go on forever.

How do you like that theory?

You have assumed Ayleid Guardians but the OP said forts and caves as well.
You have forgotten the most obvious answer, the migration of Necromancers.
There is ingame evidence of the movement of enemies. Azani Blackheart's, movement to Atatar.
They would move from cave to fort to ruin etc, leaving their creations behind.

holes in cave doors.


ayleid guardians under IC are perfectly preserved as if they were frozen in place.


indeed

If your theory is correct, I would like to know why the Ayleids cursed their cities, how they knew they would be plundered, or if even they knew the cities would exist after their disappearance.

I would argue about your step 2. http://www.imperial-library.info/mwbooks/legions_of_dead.shtml seems to indicate that the different kinds of undead are completely distinct from each other. Besides, zombies never carry weapons - why would they just start carrying around weapons when they "rot and become skeletal"?

Indeed, I would love to just think that. But, I'm not that cool. Being hypocritical as I say this, we have to give some background to Oblivion. We must savor the few beneficial questions o' lore Oblivion did give us.
to the point; There is a definite several factors that contribute to the presence of undead- But I'm going to only talk about the ones that do have to do with lore, which cuts the number down to a few:

1. Necromancers. Necromancers are always in ruins and other interiors. Ayeleid and fort ruins have only a greater purpose of the deathly mages to lurk inside those abandoned halls. First off, in the first era, which is presumably when the Forts were built, (this is all off my head, so forgive my errors)-In the first era, Tamriel was the Arena, Me'thinks. The only reason for the Arenians to build such forts were probably to get a good defensive position. Defense=war/conflict, war=death 'n corpses. As for the Ayeleid ruins, the Merethic era probably.had some wars too, and, as before, war= death and corpses. In addition, Tombs are a popular feature to both Ayeleid ruins as well as forts, and as all of you know Tombs are where you put dead people, or corpses. All of those factors above contribute to the dense population of Necromancers in forts and Ayelied ruins. And, Necromancers tend, as is their namesake, to make the dead alive again. Undead, to put it bluntly, and then the Necromancers leave their manipulated walking corpses to stray and wander the forts and ruins.

2. As Dogsbody said, some spirits are bound to mundus. Some may stray and wander for eternity. Others maybe restless and appear as ghosts and Wraiths. the remaining choose to manipulate a carcass or two.

3. Pretty much everything that Axussridare mentioned. Bandits and other creature wander into the ruins. When they die, you pretty much go through way I said in to previous factor(2).

Liches, on the other hand, are a complete different story. Though game machanics state that Liches are a high level leveled undead, they actually have some differences all together compared to their mort-fleshen and skeletal cousins. A Lich appears as an undead, and, in some ways, are undead. Although not a term, the Lich may be considered Unliving-dead.
What I'm trying to convey is that Liches are living dead, but they didn't go through the process of dyeing, then being reanimated, therefor not undead. Liches are theoretically elite Necromancers in they form of walking dead, they have nothing to do with factors 1,2, and 3(well, sort of related to 1). The only real way to depict whether or not Liches are truely undead is to look at the process of becoming a Lich. Even with that looked at I could go on for hours typing a philosophical lecture on whether they're dead or not.
As there are many versions of the process to lichhood, Bethesda has not officially released the correct way to become a lich in TES's perspective. Though in my opinion, I believe that M'Aiq is correct:

"You wish to become a lich? It's very easy, my friend. Simply find the heart of a lich, combine it with the tongue of a dragon, and cook it with the flesh of a well-ridden horse. This combination is certain to make you undead."
Although it is hypocritical, as M'Aiq refers to a Lich as being undead. Though he does not specify whether or not to eat the concoction.(and yes, I'm aware of M'Aiqs conceled joke in the dialog)




Sorry, I tend to trail off, but never really this much...

That is possible, but why would a Necromancer want to leave his/her carefully raised army behind when he/she most likely can bring it with him/her?
And why would they even leave? They find a place, they stay and raise their army, conduct their research and so on. Then what? A group of paladins find their lair? The undead would be destroyed. Anything that could threaten the necromancers to leave would most likely threaten the undead as well.



Why would anyone want to loot a tomb full of treasures? In real history there are examples of traps being made for tombs. The Egyptian pyramids are an excellent example. Of course the Ayleids could have forseen the greed of man.



Let me put it a different way. The zombies are sure built up a different way. So when they rot they might "die". The "curse" would then raise them as skeletons. They are bound to the tomb forever. There is most likely a difference between the undead raised by a curse or Ayleids and the undead raised by modern necromancers. Why zombies don't carry weapons is a mystery to me. Perhaps their rotting hands are to unreliable (skin might fall of when they swing a weapon so that they lose their grip). There is another good answer.

Who says undead don't polish their weapons when you're not looking?



Well they still have to go out shopping and stuff, don't they. Many of them probably hold down a 9-5 job too.

I think there's a myriad of answers available to the Ayleid ruins. As to which answer specifically Bethesda might consider the best one, we may not know, but I think those options were well enough to satisfy the gapping hole of curiosity.

Perhaps the question now is that of the forts... plenty of corpses could hang around from said wars, but their reanimation...hmm. Necros would be the best answer in my opinion, but perhaps this would be more sound, as there does (at least to me) appear to be less saturation of the undead within forts.

Seriously. <_<

At least in Morrowind they gave a reason for undead infesting Tombs...the natives created them as guards on purpose.