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Primum Castellum, also known as the First Human Village, was the first civilization of Humans, formed not too far away from Mount Ebott (About 5 miles away). It was the largest city and the capital of the Human Kingdom for a long time, until recent events occurred that caused it to become deserted.
A very long time ago, far before history was even being written, Primum Castellum was formed by a small tribe of Humans, who wished to set aside their primitive ways for progression. This resulted in Humans beginning to become civilized, only a bit earlier before Monsters were doing the same thing. This resulted in new concepts being sparked, such as trade and history. And as a result, Primum Castellum ends up being the largest city on Earth for a very long time.
As time passed, there has been many monarchs to rule over the Humans. Most were good, some were decent, but there wasn't any that were necessarily bad. There has also been several cultures being tested in this area, until it was determined that Roman culture was the best for the area. At the peak of Primum Castellum in ancient times, it was ruled by Emperor Lucas the Third and Queen Clementia, with trade being prosperous and interactions with Monsters being even more peaceful.
However, this wasn't to remain for so long, for Lucas had died of the Plague. Even worse, tension began to grow between Humans and Monsters. It was realized that Monsters could absorb Human Souls, and becoming infinitely more powerful over Humans. This caused a limitation in the number of Monsters that could be in Primum Castellum at a time, which wasn't too bad. However, the fire was lit when the parents of Natis Prosec were killed, and it was thought that a Monster killed them. Asgore, King of the Monsters, had denied this, and the two races had gotten into more heated conflcit.
Eventually, the great War broke out between the two races, and it was quite a time of depression for Primum Castellum. Most of the males were gone, as they were serving in the army. Queen Clementia was in a state of inactivity, not really making any orders for the army. Finally, the War ended with the Monsters sealed away. The Humans however lost one of their finest to an impalement by Asgore... Natis himself.
Times after the War were difficult, and most Humans traced this to the inactivity of their own queen. Furious with leftover-anger from the War, the Humans had decided to overthrow Clementia, throwing her into the same cave that the Monsters were locked in. In place, the Democracy of the Humans was appointed, and the civilization was soon back in business. Trade was back up to its standards with newer technology, and entertainment was at a new peak.
Over time, Primum Castellum was eventually declining from this peak. It was soon no longer the largest city or the Capitol of the Humans, for the city of Falmouth had taken both titles many years later. Soon enough, Primum Castellum was just the nostalgic city, even if it still had a rather large population. However, even the rest of the population fled, when the newly-made Robots in the city had decided to go rogue, causing the city to lock up and evacuate.
Primum Castellum was then only used for the Humane Labs, which was the only stable building in the city around the time. A rather interesting test otherwise known as the Reincarnation Experiment had occurred about ten years ago, testing a SOUL essence known as the "Forgetting". However, this test had soon compromised this facility as well, when it caused a test subject known as Marcos Raw to attack the scientists, and become the Dark Corpse.
Now, Primum Castellum is a city lost in time. Overrun by the present and the past, this city has a current Human population of zero. It has been locked up and quarantined by the order of the Democracy of the Humans, and only one with an ancient Heart Key will be able to enter such a puzzle once more.
Primum Castellum is a very large, yet abandoned city that is shielded by a purple dome. This purple dome is actually a magical barrier similar to the one that encases the Underground, and has been utilized to protect the city from dangerous attacks and storms.
A large door is the only entrance into Primum Castellum, which is locked by a nearly indestructible heart-shaped padlock. In order to unlock the way in, one must utilize the Heart Key to unlock the padlock and enter the city.
The buildings range far and wide in terms of age, size, and condition. In terms of age, they can range from before the War, and to about 20 years ago. In terms of size, they can range between small stands to gigantic skyscrapers. In terms of condition, they can range between nearly untouched to absolutely destroyed.
Agroid was the first robot created by the Humans, and was a very powerful influence during the War. After the War, Agroid mostly stuck around to entertain the Humans. He was eventually deactivated however, and remains dormant in the Colosseum, waiting to be charged up. Agroid is the main boss of Primum Castellum, unless he has already been activated and has gone into the Underground.
While Primum Castellum may be abandoned, Guard Robots are scattered around the entire place, coming in several different varieties. They are the common enemies of Primum Castellum, and are mostly spared by Motivation and Disabling.
The Ramsington Palace is a large yet slightly untouched palace that is located near the east end of Primum Castellum, in the rich district. It used to be the capital building of the entire kingdom, and the home of the great royal family, until the capital shifted somewhere else. The palace can be explored to look for many undiscovered treasures inside, but is heavily guarded by some of the older Guard Robots.
The Augurium Aeger is a large, ancient mansion located somewhere along the southwest section of Primum Castellum, and belonged to Arkar Elymas, the Archmage of the First Human Army. It has an incredible, almost otherwordly, magical aura to it, encased in a flowing, bright purple barrier, coming from runes made around the whole house. It is damaging to those unworthy of entering.
Inside, there are many ancient decorations, like paintings of past archmages, enchantment papers stuck on walls, a few skull trophies here and there, a shit ton of books everywhere, and a single, black-and-green orb floating in the middle of the second floor, having a clear magical aura. This orb serves as Arkar's council, and those who don't have Arkar's blood are deemed unworthy, being damaged and, most likely, killed.
The place is mostly guarded by familiars, atronachs and outerdimensional creatures, but only from the inside of the barrier, since the outside is already heavily guarded. The Augurium Aeger can be visited if Arkar is with you.
Old Home is a large cottage located to the far North section of Primum Castellum, and was the home of Natis Prosec. It houses many paintings, weapons, pieces of armor, and books. While the place is unguarded, it is mostly inaccessible until the ends of certain Routes. In here, the story of the War is explained in a different way. In Undertale: The Revival, the protagonist is often flashed here to read a journal describing the events before the War in Natis's POV.
The Colosseum was the training grounds for the First Human Army, and is located in the West section of Primum Castellum. Between the War and now, the Colosseum used to be a source of entertainment, particularly sports and fights. The Colosseum is mainly abandoned now, only to have the dormant Agroid charging in the very middle.
The Humane Labs are a complex of buildings connected together near the entrance of Primum Castellum, and was the host of the infamous Reincarnation Experiment. It has held many experiments however, and it was the largest complex before people started moving out. Now however, it has been mostly cleared, only to have a few damaged Guard Robots here and there. A bit of the Dark Corpse's backstory is explained in this building, and an elevator is available that actually leads all the way down to the Underground. It is only accessed by having Agroid place a portal that leads into it.
Monolith Tower is the tallest skyscraper in Primum Castellum, located in the direct center of the city. It was the building for many different businesses, and has been a notable piece of architecture like the other important locations. Heavily guarded with Guard Robots, this area seems to be mostly optional, except for the sidequest that Agroid may give to you...
Credit goes to TobyFox for making Undertale.