Kingdoms are essentially large alliances. A Kingdom is created when any alliance reaches level 3 and captures a Wonder. Upon doing so, the alliance leader can opt to form a Kingdom.
Kingdoms are named independently of their forming Alliances. A Kingdom can hold up to four (4) different alliance. The alliance by which the kingdom formed is entitled the Leading Alliance. The leader of the Leading Alliance is entitled as King. This title can be used in lieu of the Sire's standard titles. In addition, the Kings of Kingdoms will have the title "King" preceding their names in World, Alliance, Kingdom, Battlefield, Horn, and Trade Chats.
The Alliances that join the Kingdom upon its formation are considered Vassals. The leaders of these alliance do not adopt any new titles. In fact, all Kingdom Alliances ultimately operate independently of one another and the Kingdom. The exceptions to this condition include shared Kingdom diplomacy and open territory. Shared diplomacy refers to the Kingdom Alliances sharing the friendship, rivalry, and neutrality diplomacy held by the Lead Alliance. Open territory refers to the alliances of that Kingdom being unaffected by neutrality Alerts and barriers, much like they would within their own Alliance.
Kingdom ranking is determined by Wonder ownership. The more Wonders a kingdom owns, the higher its Kingdom Rank. Each Kingdom Alliance's Wonders are summed to determine the Kingdom's total Wonder count. However, Wonder occupation and rewards are unique to the occupier and their alliance. The Kingdom gains no greater benefit beyond ranking from wonder occupation.
In addition, although Kingdom members have friendly diplomacy (due to being in the same Kingdom), Kingdom members of different Alliances (within the Kingdom) can re-capture Wonders from one another. For example, consider the Kingdom Alpha which is comprised of Alliance Nox and Alliance Sol. The members of Alliance Nox can freely recapture Wonders belonging to members of Alliance Sol. The method of recapture is akin to capturing a wonder from a neutral or rival player, meaning the wonder will be successfully recaptured after one round of assault.
Kingdom leadership is unique to the Leading Alliance. As leadership changes in the Leading Alliance, so does kingship within the Kingdom. Should the Kingdom seek to change the king to a new alliance altogether, the Kingdom must disband and be reformed by the Alliance that seeks to hold kingship.
The leader of the kingdom can send special edicts (Kingdom Edicts). These letters are similar to the Alliance mail, but are received by all members of the Kingdom. Similar to the alliance mails, any replies to Kingdom Edicts will default to the King.
The military power of the Kingdom is separate from the Kingdom's ranking. This power is determined by the summation of power for the alliances within the Kingdom. Because military power and wonder ownership are used to determine separate rankings, it is possible for a Kingdom to have more power than the highest ranked Kingdom, simply because that Kingdom has more wonders.