Lunaria. The name makes one think of something related to the moon, maybe a kingdom that has a special connection to lunar cycles, like its magic being stronger during a full moon or its architecture being built to honor the moon.
For a fantasy story, you might consider 'Whisperwind Kingdom'. It gives the feeling of a place where the wind seems to carry secrets. 'Moonbeam Realm' is also quite unique, making one think of a place that is somehow connected to the moon's light. Then there's 'Sunspark Dominion', which could be a place full of bright, warm energy, perhaps where the sun has a special significance.
You can combine words related to nature, like 'Mysticwood Kingdom' or 'Crystalpeak Realm'. Also, think about using archaic words, for example, 'Avalon' or 'Camelot' are classic and can be a great inspiration. Another way is to play with emotions or qualities, such as 'Valor Kingdom' for a brave one.
You might consider 'Verdantia'. 'Verdant' means green and lush, which is perfect for a kingdom with lots of forests. It gives the image of a place filled with tall trees, rich wildlife, and a fresh, natural beauty.
For a castle story, 'Highcrest Kingdom' could be a great name. 'High' can suggest a lofty position, perhaps on a hill or mountain, and 'crest' adds a touch of nobility. 'Silverwood Kingdom' is another interesting choice. The 'Silver' might imply wealth or something precious, and 'wood' gives a natural and rustic element. Then there's 'Stormwind Kingdom', which has a dynamic and powerful feel, as if it endures and thrives in the face of storms.
You can start by thinking about the setting. If it's a mountainous area, names like 'Rockridge Kingdom' can work. Or if it's near the sea, 'Seacliff Kingdom' is good. Also, consider the history or legends in your story. If there's a famous hero, you could name it after him, like 'Arthurian Kingdom'.