Moon Kingdom is a location in Sailor Moon Crystal.
The Moon Kingdom of the Moon was home of Moon people and existed during Silver Millennium. Ruled by Queen Serenity, it was located in Mare Serenitatis (The Sea of Serenity). The Moon Kingdom was also home of Princess Serenity and her four guardians as well as Luna and Artemis. There was also connection between Moon and Earth, but it was forbidden for Moon people to go there.
Royal members of The Moon Kingdom bear on their foreheads symbols of small, upright crescent Moon, but civilians do not have one.
The Moon Kingdom, though it had a fairy-tale appearance, demonstrated an advanced technology. It existed within a dome which produced an artificial climate, and had an advanced computer called the "Eternity Main System". According to Artemis, ice skating was a popular sport. The original anime showed Princess Serenity being very good at ice skating. Those who resided in the Moon Kingdom were long-lived beings and had two specific duties: First, they were to protect the Silver Crystal, an extremely powerful "holy stone" which had been handed down through the generations. Second, they were to watch over the evolution of Earth and protect it from any negative influence.
When Princess Serenity fell in love with Prince Endymion, out of jealousy and under the influence of the evil Queen Metaria, a sorceress named Beryl raised up the citizens of Earth in a war against the Moon. As a result, the Moon Kingdom was completely destroyed, and Earth's history had to start all over again.
The Moon Kingdom and its castle are resurrected at the end of this conflict, but Usagi decides to continue living on Earth, with her parents and with Mamoru.