Most slimes are common monsters, found out in the wilderness of the world. They often appear in places rich in moisture, though what exactly causes them to form is largely unknown. (Someone somewhere probably knows it though.) Their bodies are naturally amorphic, taking on whatever shape gravity and nature make most convenient. They're simple creatures that live on instinct, absorbing whatever they can, then dissolving it inside their bodies to serve as nourishment. This material allows them to grow in size, until eventually they split in two, becoming another slime that squirms off to do the same thing. There's tales of a strange land where slimes began to respond to attacks from humans, taking on more and stranger forms in order to hunt and defend their home...but nobody's ever come across that place and come back to tell about it.
All that considered, how could there even be a slime kingdom? Simple: one of them got smart. It's actually a mutation - an error of sorts, where a slime ate and grew yet never divided. This process continued until the slime was roughly the size of a young dog, and that's when it discovered that it could do more than just ooze around. With all that material to work with, it somehow managed to change its form temporarily. It was still a squishy and easily defeated slime, but it was a bigger, heavier, mobile one. Slow, but mobile. The details of the story grow hazy here, since the only ones who know it are other slimes and they're not always the sharpest needles in the haystack...but according to one version, the slime became sentient due to the curiosity of a human that discovered it. In trying to learn what made this slime different, the human inadvertently taught it to think about itself, to learn and question instead of just living on instinct alone. Was there magic involved? Possibly; some versions of the tale say the whole point was to make a 'smart' slime, though nobody seems to know why.
Time passed, and this unusual slime continued to grow, until it became the current Slime Queen. There's only ever been one queen, according to the slimes themselves; differences are caused by the body's form changing, not any shift of who the body is. The Queen of the Slimes is also the substance that makes up the kingdom of the Slimes. Growing massive, able to form buildings and paths and bridges out of her own bulk, this slime's become a home for other slimes, and in turn those slimes provide the queen with food and information of the outside world. Like the queen, these slimes are intelligent...for slimes, anyway. They can change their forms based on what they need to be at the time, at least temporarily, and tend to live carefree lives where they aren't hunted by other monsters or plucky adventurers. The streets of the slime's homeland are lightly hardened, like the top of a gelatin dessert layered over itself 100 times; people walking across it might compare it to striding across a less-jiggly version of a waterbed. But most things are still somewhat transparent, so when the sun rises, the light shining through the windows and walls of buildings is a soft metallic blue that makes the place look as if it's at the bottom of the ocean.
Actually, the location is deep within a forest, a place that wasn't discovered by humans for a long time. When they got there, the slimes were already in the town, walking about in humanoid forms once they saw the newcomers. Of the shapes slime-people can take, though, humanoid is a difficult one...all long upright body and separated limbs and wiggly digits. Slimes, who have membranes but no skeleton or muscle, struggle to hold that kind of shape for any significant time. So it comes as little surprise that the Princess of Slimes is one who has this ability, taught to her by the queen herself. The current princess was once part of the queen...who can't divide, as established before. So how'd she become separate enough to travel? This was done as a special ceremony, in order to have a representative of the queen who could meet with other nations on their own grounds. She knows everything the queen knows...as of her 'birth', or the day when her body was formed and separated. Since then, they've been individuals, but gladly share knowledge with one another when they meet. She's been practicing to take humanoid form for a long time, but even for her, it's usually easier to be slime-shaped below the waist. At those times, she just 'walks' about as a torso rising from her cozy puddle, or sits in chairs with her lower body turned blobby and uncertain beneath the table. This seems to make people uneasy. She tries to keep herself with legs and clothing and all that, but man is it hard for hours and hours while her attention wanders and ooooh what's that shiny thing taste like?
Oh right, another important thing to know about slime-people is that they still have the urge to 'eat' things. The inside of their body, especially at the core, is acidic; anything that stays there long enough will slowly dissolve. (And if the slime in question either doesn't know how to, or forgets to, make their outer layers thick and semi-opaque, everyone watching gets to see that. Fun!) At the upper layers, the acid only manifests as a slight tingling...though it really isn't suggested to leave one's hand inside a slime even then, just to be safe. In any case, a slime who finds something to be tasty will grab it with some part of their body and wrap around it tightly, forcing the item past the membrane and into the body, where it slowly sinks into the core. Slimes will eat anything, too. Food items, plates, silverware, glasses, napkins, tablecloths, jewelry, books, weapons, you name it. What matters is the substance it's made of rather than what it's for. Protein is good for growth, metal for physical protection, wood products for vitamins, citrus to help with digestion speed, and so on. When dealing with something they don't know, many slimes will want to 'eat' it to learn what it's made of...and of course, there's no getting it back then. Slimes who are used to dealing with humans can suppress the urge for a while; just be sure to give them other snacks. They can only dissolve so much at once and start feeling 'full' once their bodies are too cluttered with things to digest. They also won't eat anything larger than half their own body in size, unless it's an attempt to defeat an attacker.
Speaking of which. Slimes don't age or die in the way that humans do. From the minute it takes shape to the moment it ceases to move, slimes pretty much operate in the same way. Intelligent ones will consider their 'birthday' to be the day that they (or someone else) named them and they were able to recognize the name as their own, but wild slimes don't really have birthdays or a concept of 'age' at all. While they can't starve, technically, slimes do need water to continue to function. Not too little, not too much; a slime thrown in a deep lake will become over-hydrated and begin to break apart. A slime with no water will begin to shrink until they've crumbled into dust. It's also possible to kill them by doing many things that would kill a human, outside of organ damage and suffocation. Slimes don't breathe, but they are very vunerable to squishing and to being torn through. A warrior can kill one with a sword not because they can bleed, but because breaking them up into tiny pieces makes it too hard for them to reform before their substance soaks into the ground, and that destroys them too. In the case of an intelligent one, being broken into small parts and not allowed to reform turns the parts into individual slimes without intelligence...why is a mystery, and none of them really want to get to the bottom of it.
So, after all of that, why would anybody even recognize a slime kingdom as being a kingdom? Well, slimes can go almost anyplace, and in their wanderings, many slimes find interesting things. Items of legendary or magical nature don't dissolve inside of a slime with the speed of other items, and they can just pick these up and carry them inside, no problem. Slime gel - the more liquid and squishy stuff between the exterior and the core - also has interesting properties. When stretched out and allowed to harden, it becomes like colored glass, but tougher. Some slimes create artwork with it, and their kingdom is full of mosaics and pictures made that way. When put in jars and mixed with certain herbs and other plant materials, it's good for long-lasting potions or medicines...if you can get over the idea of what you're drinking. If blended with other materials, it makes for vibrant dye that doesn't fade with washing. Some people even say that slime gel tastes good, all by itself, that the different colors of slimes have different flavors...but those people are seen as pretty weird. As for the slimes? They don't really mind that their bodies are used that way, since they secrete goop just while moving around in the first place. It's painless for them, like getting a haircut is for a human, and the trade allows them to get things they couldn't have otherwise. Though sometimes they have to be reminded not to eat other kingdoms' currencies... Also, intelligent slimes are capable of communicating with normal slimes. If a town's having trouble with the wildlife, a member of the Slime Kingdom can come and get to the bottom of the problem without anyone having to fight. Sure, it's 'just' a slime, but for villagers armed with blunt weapons, it's a minor monster that gleefully ate their staff after it bounced off a few times.
Slimes are a simple people who like food, sleep, friendly people, new things, and stuff that looks pretty. As a result, their kingdom is laid-back and easygoing as well. Most carry their belonging with them when they leave - often by putting them in a bag and then absorbing the bag, only to keep it at a shallow level of the body - and don't mind it if someone's taken up residence when they come back. The queen's court has absorbed behaviors and traits from humans and other species, so they have their share of snobs, gossips, and hangers-on...but for them it's like a fun game, on some level. Even the princess, who travels the most outside of their territories, has to be reminded of the importance of things she doesn't 'get' due to her nature. Matters of food though, those are always important! Weather changes too, and incursions of powerful monsters.
...Ah, it's almost 2am, no wonder this has gotten awfully ramble-y and messy. Apologies!