I envision the AK as a single giant continent (hence Pangea) covering from the Paradise to half New World. Mariejois, pre-castle, was one of their highest mountains. When Imu attacked the AK, the continent was submerged, except the hills and mountains. Mariejois, being the tallest point available, was taken over by the 20 kings to build their castle and save themselves from the flood. THEN the Red Line was artificially build -- after the AK was already underwater.
But the mountain Mariejois lies on was already important before the CDs. The Eve Tree already tells us why. I think, much like the giant mountain trees we see in the background in Elbaf, Mariejois was one of the two Trees of Life of the planet. One belonged to the AK. One to the giants. The one belonging to the AK was chosen by the lunar people as a contact site when they descended to earth. It was a place high enough they felt safe to land on, and could easily defend from potential aggressors. Thankfully for them, the AK were a pacifist bunch, so a trading deal was agreed upon. The AK would give the lunar people resources; in exchange, the lunar people would give them knowledge. Thus, Mariejois became the land of the gods (they knew so much more), and the AK flourished.
But then something rubbed Imu the wrong way, and the world was changed forever-ish.
My personal head cannon is kind of like the following:
Originally, the Human Kingdoms were kind of like the Human Kingdom in Toriko. You may not have read that story before, so I'll explain. In Toriko, there was a Human World and a Gourmet World that surrounded it. The Human World was relatively small compared to the Gourmet World, and the Gourmet World was unexplored.
In One Piece, I think the Ancient Kingdom was originally a Human Kingdom. Joyboy was a Human King. However, Joyboy saw that there was an entire world that was unexplored full of mystery and adventure. This was why he became known as the first pirate - he left the "Human World" and started to explore the "New World".
When he left the Human Kingdoms, he discovered the territories of the ancestors of the Mink tribes (Terrestrial animal men) the Fishmen (Aquatic animal men), and the Bird tribes (Aviary animal men).
Joyboy was especially good at Haki and gained new insights about it as he journeyed.
The main part about that is... Joyboy was a Human King who introduced Human Tools to the Animal Kingdoms. The inhabitants of the Animal Kingdoms were actually previously humans, too. However, over a long period of time, their Haki became integrated with their environments. People who especially liked the Land became Mink Tribes, people who especially like the Sea became Fishmen, and people who especially liked the Sky became Bird Tribes. They evolved.
When Joyboy encountered these groups, they were already constantly warring with each other. Actually, the Animal Kingdoms were the original three great powers. Similar to the Warlords of the Sea, the Emperors of the Sea, and the World Government. Except, they were all in equal standing.
The Animal Kingdoms had tools, too, but they preferred to use Natural Tools like their beaks, talons, claws, and whatever else they may have had. However, they each had challenges that made it nearly impossible for them to work together. When Joyboy encountered them, he helped them to overcome these challenges using his Human Tools and insights about Haki. In return, each of the Animal Kingdoms shared their treasures and insights about Haki with him.
As a sidenote, I want to call these Environmental Haki types to be called Haki Affinities. For example, if a Human played in water all his life he would gain a Haki affinity toward the aquatic environment. These Haki Affinities would later take the form of Fishmen Karate, Electro, and Mantra.
Long story short... Joyboy became the World King by uniting the Animal Kingdoms because they were the most powerful organizations on the planet at the time. Furthermore, the names Poseidon, Pluton, and Uranus refer to the Ancient Kings of the Fishmen, Minks, and Aviary tribes who were revered as Gods. The Ancient Weapons were created through the use of Human Technology and mastery over Haki Affinities (each one corresponds to the Land, Sea, and Sky).
When the Human Kingdoms discovered that Joyboy had become the most powerful Human King, they plotted to overthrow him. This is when the Great Alliance attacked the Ancient Kingdom. Further, I believe that the Animal Kingdoms and their kings were Joyboy's friends. Finally, the reason why Joyboy got killed back then was because one of his friends betrayed him - it was Uranus. Furthermore, Princess Vivi of Alabasta is Uranus' descendant, and she has a dormant power inside her that will awaken and allow her to control monstrous birds similarly to how Princess Shirahoshi can control Sea Kings and how Momonosuke can control Zunisha (see a pattern here?). However, Uranus was cursed as a result of his actions, and it caused the Nefertari family line to lose their connection with the sky.
Pell was not allowed to let Vivi ride on him because it would awaken her interest in the sky, again. Furthermore, Pell solved a certain problem, too. By allowing her to ride him, she became obsessed with flying, but at the same time, he taught her the correct reason for fighting. Why did he teach her that? Because of the same reason Princess Shirahoshi could destroy the world. It is said that the Ancient Weapons can either save the world or utterly destroy it and it would depend on a person coming to them and guiding them toward the correct path to take. Pell taught Vivi that he doesn't fight because he likes it, and that he fights because he is protecting people he loves. His sacrifice in Alabasta cemented this (he carried Crocodile's bomb into the sky and it looked like he died). This weird little thing about Vivi is also why she had birds that couldn't fly before. The duck-billed squad guys, and Karoo.
The amazing technology of the Ancient Kingdom was developed by meeting the Animal Kingdoms who each had innate knowledge about their environments. After sharing that innate knowledge with Joyboy, he had gained innate knowledge about ALL THREE major environments or realms if you'd prefer to call them that. Originally, I called the Ancient Kings [Realm Kings]. In any case, Joyboy knew everything about the Land, Sea, and Sky as a result. That knowledge is what allowed him to become the most powerful person on the planet because he basically gained innate knowledge about the entire world. It's how he gained The Voice of All Things.
To me... The Ancient Kingdom and the Lunarians are the same people. The Lunarians are evolved humans who went up to the moons after the fact (and then came back down later).
As a final side-note, Pangea was only one part of it. Do you know what Pangea means? It means All Land. Do you know what the sea is called? Panthalassa. It means All Sea. Then, there is ouranus (Uranus) which is the sky, but it's not referred to as panouranus (Panuranus) because it's always in one piece. It doesn't get broken up like the land or the sea does.
The All Land = Pangaea = Minks lived there
The All Sea = Panthalassa = Fishmen lived there
The Sky = Ouranus = Birdmen lived there
Did you notice the other thing here? The All Blue is Panthalassa! It is the ocean that surrounded Pangaea.
Considering all this... There is no need for a special moon race.