Each kingdom has three requirements in order to be unlocked: player level, gold, and unlocked neighboring kingdoms that connect with them.
Kingdoms may have a player level restriction depending on how far they are from the center of the map, up to Level 25.
All kingdoms cost gold to unlock. The cost to unlock a kingdom increases based on how many kingdoms have already been unlocked, up to 4,000 gold.
Kingdoms can't even be detected without the player unlocking a kingdom that borders them, so in order to unlock a kingdom, players must first unlock at least one kingdom that is nearby.