In Age of Empires: Castle Siege, the player maintains and develops a castle that produces resources, which can be used to train and upgrade buildings and troops, as well as research new technologies. The trained troops are used to attack other players' castles in order to loot their resources and to earn crowns, the game's scorekeeping mechanism. Upgrading buildings is needed in order to generate resources faster, hold more resources, defend the castle against enemy attacks more efficiently, and unlock new technologies. Researching new technologies grants multifarious advantages to the players, such as additional crowns in case of victory, and reduced resources that can be looted from the player from each attack. Players can also form alliances with each other for various social and strategic functions, such as supplying each other with relief forces, which help players to defend against enemy attacks.
The game features a PvE campaign mode, titled Historical Challenges, that puts players in a series of historically based scenarios. There are ten scenarios, each requiring the player to reach a specified Age before being unlocked for play. The scenarios are the following:
- Sack of Thessalonica
- Capture of Aleppo
- Siege of Dorostolon
- Siege of Constantinople
- Siege of Kenilworth
- Siege of Acre
- Siege of Calais
- Siege of Marienburg
- Siege of Kaunas Castle
- Siege of Orléans
The player can choose to play as one of six different civilizations:
- Kievan Rus
- Teutonic Order
The castle is built around a keep, surrounded by buildings of various types. Economy buildings produce and store resources, while military buildings are used to unlock, upgrade and train armies. The castle is protected from enemy attacks by fortifications, including walls, towers and various traps. Players can also build decorative constructions that serve no practical purpose. All buildings can be updated to increase their production and to gain additional perks. Updating the keep increases the number of buildings allowed and makes additional building types available. Sometimes players are provided with some "boost buildings" which can boost the production rate of production buildings or training rate of army camps, such as "medieval fair". These "boost buildings" don't need to be destroyed by the player to earn stars in battle.
There are five different resources in game. Food is produced at farms, and is mainly used for unit training. Wood is produced at mills, and is mainly used for construction and siege unit training. Stone is produced at quarries, and is used for building and upgrading defensive structures. Pennants are produced at Universities and Cathedrals, and is used for researching new technologies and unique units. Gold is awarded in daily gameplay or purchased with real-life currency in microtransactions. Gold can be used to speed up the production and research processes, or to buy other resources.
Units are separated into four types: infantry, archers, cavalry and siege weapons. Each civilization also has its own special unit, which are more powerful than the basic units. All units can be upgraded to make them more powerful. But by upgrading, the cost of training will also be increased. A maximum of two heroes can be taken into combat when attacking. Each hero has a unique skill, and all heroes have a very high HP and DPS, so they are the most powerful units in the game. Once the player has built the Hall of Heroes, the player will get the first hero of their civilization for free. Players can unlock and upgrade heroes using pennants in the Hall of Heroes.