The castle is the main building in the kingdom and the only building a kingdom starts with.
At first, the castle consists only of a Throne Room but, by spending resource points, a Barracks can be added to it. After several quests have been completed and the kingdom begins to gain renown, a Reception Hall will be built automatically. Once the reception hall has been built, resource points can be spent on building a Spy Quarters.
The Throne Room
The Throne Room is the main area of the castle and is the home of the Monarch. It consists of the titular room, a parlor for playing cards and reading, a study for strategizing and writing, a kitchen and dining room, a bathroom, and the monarch's bedroom. Players that pre-ordered their copy The Sims Medieval or purchased the Prima Official Guide received special codes which can be redeemed via the "..." button on the main menu; among other things they provide variants on the standard throne room when starting a kingdom.
The Reception Hall
The Reception Hall is essentially a very large dining area. Along with many tables and chairs, it contains several casks for brewing drinks and a kitchen area with both a cooking pot for stews and an oven for baking.