Castles II was developed by QuickSilver and published by Interplay. Gameplay includes scouting out unknown territories, conquering them, building castles to prevent revolts and line defenses, raising an army, feeding and paying them, and eventually making a claim for the title of King. Depending on how strong the human player or the other four AI-engined nobles are, the Pope will decide whether or not to endorse the claim. Therefore, attacking someone that claims the title can prevent them from getting it. Using diplomacy also allows the player to maintain high relationships with the other nobles and with the Pope, a useful feature to protect from attack. Alternately, the player can conquer everyone and even the Pope to win by default (in which case the anti-Pope will also endorse the player's claim to the throne).
Depending on which noble is picked, the player can start at any of five general areas of the map. Initially he is provided with one territory rich in one of four resources: gold, timber, iron, or food. Having more of one kind of resource territory increases the total amount that the player can harvest per turn (also dependent on the total amount of "points" allocated). Players can only gather the resources in territories they control, so a player who does not control at least one of each kind must rely on trades to gain those resources.
At first the player can perform one task each of three types at once: administrative (gathering resources and building castles, represented by a green bar), military (recruiting an army, building optional weapons like a catapult, and policing the realm, represented in red), and political (sending scouts, diplomats, and spies, represented in blue). The more a type of task is performed, the more points which may be devoted to that kind of task are gained (to a limit of 9 per type). At a rating of 5, two tasks can be performed at once.
The most remarkable feature at the time for Castles II: Siege and Conquest was the ability to design and save different castles. Depending on the total number of walls and turrets, the castles were assigned point values that determined how long it takes to build. Larger castles are harder to destroy or capture, which serves to keep enemies out of the controlled lands. Large castles are also used to prevent revolts. Armies include infantry, archers, and knights, each costing a different resource to recruit. The size of the army that can be raised is dependent upon the number of territories and castles the player possess.