What is the ruling on the online strategy game “War of Kings,” which relies on developing castles, armies, and spies to attack and spy on other players and steal their resources to level up, with a system of in-app purchases for greater advantage in the game?

It is not possible to rule on the specific game mentioned, but generally, if games are free from شرعي (Sharia-compliant) prohibitions, they are permissible; otherwise, they are forbidden. If the game is permissible, then paying money to develop it and benefit from it is permissible. One must avoid games that simulate theft and espionage, especially for children.