Uzumaki/WMG

The Spiral Abomination behind the events of Uzumaki is Uchiha Madara
Seriously, look at him! "Everything will become one with me", just as the whole of Kurozu-Cho became one with the Spiral City. The Spiral will devour all the countries, one by one. There is no escape, you fools!

Kurozu-Cho is Kirie's Purgatory
The town's curse keeps repeating because she has not yet been purified of her apathy and distrust. All she has to do to escape is to leave with Shuichi during the early stages of the spiral curse.

Spiral City is in the early stages of a Spiral Nemesis
Self-explanatory. It'll speed up and grow quite a bit once its consumed more people and area.
 * Actually it makes alot of sense. Since the Spiral Energy is out of control, the people are able to do extraordinary things that are beyond the impossible.
 * Alternatively, it's an early attempt by the Anti-Spirals to prevent Spiral Nemesis. It's a device that draws Spiral Power and the lifeforms that generate it into itself and contains them. But for whatever reason, they couldn't get it to work on a scale bigger than a small town, so they moved on to the falling moon plan instead.
 * Maybe it's anti-spiral energy. You know how spiral energy was green, right? Maybe it's dark reddish purple, just like the energy Lordgenome was using and that the anti spirals were using. Does that exist? Remember in TTGL when that beastman thingy was spinning, and made that reddish purple energy? And Simon would it in the opposite direction and it made normal spiral energy? Discuss.
 * What made you thing they couldn't get it to work? What do you thing Kittan destroyed?

Spiral City is Silent Hill.
With a geometric theme and an even more extreme horror to it. 'Nuff said.

Whatever happened in Kurozu-Cho, it's happening again...
...but in Norway this time. They tried to hide it with some kind of "russian missile" nonsense, but we tropers should know better!
 * Oh, God.

The Spiral City is actually a fragment of The demon world of Ghul in Eisenhorn. More specifically, Yssarile's barque. Thus placing Uzumaki and Warhammer 40,000 in the same continuity.
Obligatory warnings: Firstly, spoilers (untagged because the whole thing would have to be spoiler). Secondly, comprehensive evidence... which means a huge wall of text, unfortunately. You have been warned.

Let us examine the evidence from 'Hereticus', the third book of the trilogy. In Hereticus it is mentioned that Pontius Glaw has fled to the Demon world of Ghul to activate Yssarile's Barque. From the way Dan Abnett describes Ghul, we can deduce that the Spiral Abomination of Uzumaki is Yssarile's Barque. Consider the evidence.
 * Firstly, in the chapter where we get the first glimpse of Yssarile's 'civilisation' (at the auto-séance on Promody), the writing on the ghost city was said to "simply spiralled and meandered up across the massive wall face, looping and circling." It was also "sickening to look at." Shuichi himself gets sick of looking at the spirals right from the beginning of Uzumaki.
 * Secondly, when Eisenhorn has Aemos do research about what is hidden on Ghul, Aemos is nearly driven insane and the poor guy was writing out his research results... in spiralling sentences across his entire room. "His notes have taken the form of the chart itself." This fits in perfectly well with Shuichi's father's collection of spirals in his room.
 * Thirdly, Eisenhorn gives the suggestion that the surface of Ghul itself may have massive spirals. "The warped one's entire culture, certainly their language, had been built upon expressions of location and place. I imagined that the inscribed wall we had seen during the auto-séance had been part of just such a maze of lines, from a time when Promody had looked like Ghul, the capital world." Later, a small spiral is also described as "a really tiny version of the mazed surface of this planet." In Uzumaki, the corrupted inhabitants eventually rebuild the entire town in the shape of a spiral.
 * Fourthly, when the archaeologist Kenzer was making a recording of the same small spiral on one of the walls, said spiral acted as some sort of doorway between different dimensions, with Ghul being so complex that it has more than three dimensions; that only certain points can act as entries or exits. Given this, we can assume that Yssarile's Barque has been exercising the same kind of power over the singular land tunnel linking Kurozu-Cho to the outer world. As the inhabitants gradually become more corrupt, it gains enough power to create a pocket dimension. Failing all else, how do you explain a black hole appearing in a girl's forehead?
 * Fifthly, the Mausoleum of Ghul is vast beyond comprehension. The floor is also described to be made of stone. Time also malfunctions within the chamber, as Eisenhorn himself mentions. Obvious link here; the chamber of the Spiral Abomination was vast; Year Inside hour Outside in full force near the end. If Kirie's words are anything to go by, at the end Yssarile's Barque is capable of controlling time itself. Furthermore, the chamber is made with stone of petrified people. Eisenhorn may not have noticed this due to lack of light.
 * Sixthly, the interior of the Mausoleum is covered with spiralling script (not surprisingly) and something the size of an Imperial Hive city of a geometric shape. That basically describes how Kirie views the Spiral Abomination by the end of Uzumaki.

Well, that covers the basics of the link. Three more questions to be covered, though.
 * Firstly, why were the effects in Uzumaki not apparent on Eisenhorn's team? We can safely assume that this was because they were never exposed to the full power of Yssarile's Barque; only the periphery effects. All they ever had was indirect contact, and even when Eisenhorn was near it he was too busy fighting Glaw... and a singular fight with Glaw will never take up as much time as The Barque had been hiding under Kurozu-Cho. What we see in Uzumaki is the true power of corruption an artefact of a Chaos/warp god can unleash. Yes, it is not pretty.
 * Secondly, how did the Barque get under the town in the first place? Well, given how the Imperial Navy annihilated the planet (i.e. blew it up); we can say that its unique multi-dimension quality means fragments of the planet travel through space and time! Due to the Timey Wimey-ness of this, it is entirely possible for Yssarile's Barque to be underneath Kurozu-Cho even before Earth was created.
 * Thirdly, why is it the Barque that survives the Imperial bombardment? Well, it IS a remnant artefact of a warp god, and they would be savvy enough to have a plan B. For most warp gods, this boils down to finding a method to consume enough souls to regenerate. Given the amount of power it exerts over the people of Kurozu-Cho, I'd say it is well on its way to recovery.

Oh, what the heck. Dan Abnett may actually have been inspired by Uzumaki... If this theory is true it definitely explains why Glaw was so desperate to attain it as a weapon. Am I going insane, or does this make sense to anyone!? (To end on a happy note... send in the Grey Knights!)

The... thing in Spiral City was actively trying to prevent word of the curse from spreading from the very start.
Let's start with Shuinchi. Shuinchi has some sort of hyper-awareness of supernatural phenomena or whatever. And the spiral-curse-thing was aware of this. Both of his parents were eliminated at the very start to decrease his credibility, so that the curse would gain more time to manifest via everyone believing that Shuinchi's warnings are just a result of his strangeness (which only worked in the beginning when there appeared to be nothing wrong) and grief from losing his parents (the curse keeps on making him see his father all over the place). Thus, even when it should have become obvious that there was something seriously wrong with the town no one budges until it's too late.

Then we have Kirie. The lost chapter was provided to explain how Kirie was telling the story. Somehow she had retained her telepathic power (but didn't know how to unlock them or the fact that she still had them at all) and the spiral-curse-thing knew. And thus the events of the curse happened mostly around Kirie in an attempt to eliminate her early on. Kirie somehow managed to unlock her telepathic powers. The story is being told to someone else living in Kurozu-Cho in the future who is about to undergo the curse.

Kirie is wrong about the spiral-thing's motivations.
The spiral is actually a victim of the last iteration of the curse, and was trying to get as many people out of there as possible before the curse reached the Closed Circle stage.

Kirie is a liar.
Ties in with the above. The reason why things keep going wrong around her is that she is the person who, in each iteration, starts the curse in the first place. Shuinchi has some degree of mystical awareness of his own, but told the wrong person that he'd found out about the curse. Kirie is lying to him because a) she cast the spell to make him hers forever, and b)he could break the spell by killing her.

Sometime after the ending of the story, Everyone got better.
I mean, how else can the main character tell the story?
 * Perhaps people frozen in the city can communicate with one another. Which means...

The "Spiral City" below the town is either an ancient or alien machine that can change reality.
it's this troper's interprtation that whatever it is below the city is either an ancient (think lost city of Atlantis or Mu) or alien machine built there long ago that can change reality so long as it's related to a spiral and perhaps it's malfunctioning, which would explain why so many random and bizarre things happen in the town

Uzumaki was written specifically to place the "spiral obsession" in all the readers.
Think about it: How many spirals did you start noticing around you after you started reading? How many times have we said the word "spiral" on these pages? A passing fascination in the shape is just how it started for all the characters...
 * Oh, you sick bastard...

The Curse is a part of an Assimilation Plot
People who succumb to the curse are assimilated into the spiral, which holds the collective consciousness of every individual who was claimed by it.

Uzumaki is set in the Naruto universe, just several thousand years after.
Kurozu-Cho is where the Village in the Whirlpool once stood.

The movie version is partly about Kirie becoming aware of & rejecting the patriarchy.
This is a long one, but hear me out (also, this interpretation only applies to the film, not the manga): Specifically, the movie is about how a girl might want a man mainly for support & stability, but this in fact limits her. Kirie doesn't want a lover or a husband, she just wants to have a father figure always taking care of her. From the beginning we see that she treats Shuichi as her superior, talking about how he's so much better at math and playing up her own inadequacies. Even in their childhood Shuichi took on a nurturing role for her--she explicitly denies him from replacing her mother, which conspicuously leaves the potential for him to replace her father.

As I recall, Shuichi and her father never share a scene--when Kirie finally takes Shuichi to the kiln, the domain where her father would prove himself as a man and a breadwinner (the kiln not only allows him to support his small family, but it is a place of creation--reasserting the male role as a generator and controller of life), her dad has mysteriously disappeared! The only things he left behind are warped spiral pottery, symbolizing that he has become foreign and isolated from Kirie (since the spirals are always an inscrutable "otherness"). Shuichi has clearly usurped the father's place in Kirie's life (he is already calling the shots; when he declares it's time to leave town, Kirie no longer questions him), cancelling out her father's existence altogether. But then Kirie realizes that depending on men to control her life only hobbles and limits her; Shuichi becomes a monster in her eyes now that he has gained the authority and dominance of a father figure.

Thankfully, before he can start exerting his power over Kirie, she realizes what a monster she has created and how constrictive this relationship truly is. In the closing montage, it appears that Kirie has vanished from the kiln--indeed, thanks to her understanding, she has whisked herself away from her father's and Shuichi's domineering little world. (Alternatively, you could say that her fate is left ambiguous because she is dead and alive at the same time: her experience with Shuichi has killed her childish innocent self, but her jaded, cynical self will live on elsewhere.) In her closing lines, she reminds us that this is "the village of my birth"--but now that she has chosen to be an adult, she can cut the umbilical cord and leave that place to assert herself as a self-reliant individual. If Kirie gets another boyfriend, she will no doubt regard him as a partner and an equal, not a superior surrogate father. She has finally grown out of her needy, childlike stage and won't let people assume such control over her life again.

Kirie's eventual maturity contrasts with the helplessness of Shuichi's mother. In Shuichi's household, he replaces his own father as the family's center of reliability and control, almost as if rehearsing to do the same in Kirie's family. His mother has clearly never grown out of her feelings of dependence on men, and she meets a tragic fate because of it. She entrusts herself to Shuichi (he is the one meeting & planning with her doctors) and blindly hopes that things will get better. But of course things don't; her downfall is due to her lack of autonomy and her faith in the patriarchy. In contrast, Kirie becomes self-reliant and vanishes safely from the miserable town. (This also ties into any theory that the people turning into spirals are an "Invasion of the Body Snatchers"-style symbol for social conformity.)


 * Also notice how Jack-in-the-Box is so unabashed about the fact that he wants Kirie not simply because he admires her, but mostly because he would have prestige, esteem and influence. He's another menacing character who views women as just a tool to exert power over.

Kirie is telling her story to the TARDIS or the Doctor
Who else can she be talking to from a place where time has stopped? And who else is better to deal with a time bending Eldritch Abomination apart from the Doctor?