This week for our author resources we're going to look at different parts of a castle! I didn't realize how many pieces there were to a castle until I started learning about them; it was far more complex than I thought. They could be very elaborate and complicated, with many different layers intended to protect them from invaders.

This list is intended as a reference for people writing fantasy. If you're going for historical fiction, I would suggest you use this only as a starting point; there's a lot more to learn if you want it to be historically accurate! (One of my favorite sites for castle information is this one.)
Allure: An allure is also known as a wallwalk. This is the part of a castle that allows defenders, soldiers, and inhabitants to look out over the curtain wall.

Armoury: An armoury was a place to store weapons and armour, as the name suggests, and could be located in the main keep. As primarily a defensible location, castles had need of weapons and armour within reach.

Loopholes: Loopholes, or arrow loops, were narrow windows or slits that archers could shoot from while being sheltered by the walls.

Bailey: A bailey is an open space between the castle walls and the castle itself, where necessary daily business was done, such as gardening and animal tending. There may be more than one bailey, in which case they may be referred to as the upper and lower baileys, or the inner and outer, as well as named according to their direction (west bailey and east bailey, for example).

Barbican: A barbican is a form of defense, usually past the outer edges of a castle, and used as an outpost or as a defense of a gate or bridge. 

Bartizan: A bartizan was an overhanging turret on a wall, often furnished with loopholes or arrow slits. They could be square or round, and are most often found on corners.

Bastion: A bastion, or bulwark, is a protrusion from the curtain wall formed of two faces and two flanks. It could be used to protect the curtain wall or other bastions along it.

Berm: The berm is the flat area between the moat or ditch and the outside of the curtain wall.

Buttress: A buttress is a structure built to provide more support to a building, such as a building that lacks enough support for the weight of the roof, which pushes walls outward. There are many types of buttresses, such as the flying buttress.

Courtyard: A courtyard was an open space within the curtain wall.

Chapel: The chapel of a castle was used by all castle inhabitants for worship and prayer, and was often divided into two parts, one for the lord and other high-ranking personages, and one for the servants.

Crenelations: The correct term for battlements, crenelations are ramparts built along the top of walls and towers. Merlons are the solid portion of the crenelations, and crenels are the spaces between, used for firing arrows.

Cross wall: The crosswall was another architectural support system, meant to help hold up the curtain wall from the inside, or interior walls. This also gave any attackers another wall that they needed to break through.

Curtain Wall: The main wall enclosing the castle, the curtain wall was built to act as a fortification. A curtain wall was a stretch of wall between two towers, and was usually connected to the gatehouse.

Drawbridge: A movable wooden bridge meant to rest over a ditch or moat, the drawbridge would be lifted up to prevent easy access to the bailey. It was lifted using rope or chain, and fastened to the curtain wall with hinges.

Dungeon: The dungeon of a castle was used to hold prisoners, and could be located either in the towers (since they were the most secure locations) or below ground, which were also secure.

Forebuilding: A projecting fortification to protect the entrance and/or stairway to the keep or dungeon.

Galleries: A passage built into the walls of the keep, galleries could be used to allow movement around the upper levels, as well as provide a vantage point for watching the hall below and attacking, should the hall be taken by enemies.

GateHouse: The gatehouse was the main entrance to the castle, where the drawbridge and portcullis were located. Generally considered the weak point of a castle wall, the defenses of a gatehouse could be elaborate.

Hall or GreatHall: Considered the heart of the castle, the hall was one of (if not the) largest of the rooms in the castle. This was generally where banquets and meetings were held.

Keep: The central or main tower of a castle, the keep was the final line of defense in an attack of a castle. It house living quarters, store rooms, and sometimes even the main well of the castle.

Machicolations: Similar to balconies, machicolations were parapets extending over the curtain wall. They had holes, called murder holes, in the floor through which defenders could drop things such as rocks on the heads of attackers.

Moat: A moat could be filled with water or kept dry, and surrounded the whole castle. It was meant as a form of defense, and if it was filled with water was meant to be too deep for wading through. If the maot (or ditch) was dry, it could be filled with wooden stakes.

Motte: The motte was part of a motte and bailey castle, and refers to the mound of earth upon which a tower was built. 

Oubliette: An oubliette was a particularly nasty type of dungeon, different from the average dungeon because it was a narrow hole in the ground, claustrophobic, and reached only through a trapdoor high in the ceiling. Prisoners were often put in and forgotten about.

Palisade: A palisade was the forerunner of the curtain wall, and was made of wood in a type of fence.

Portcullis: A portcullis was most often used to protect the main entrance of a castle, and was a heavy, grilled door that descended vertically.

Postern: The postern gate was meant as a secret exit from the castle, and could be used to escape during a siege, or attack an enemy.

Rampart: Ramparts developed in curtain walls, but began as mounds of earth or rubble that were used as defenses. Structures were often built on top to offer a platform and protection during battle.

Stables: Stables were where the horses of the castle were kept and cared for.

Watergate: A watergate was a gate that opened directly onto open water, allowing the people of the castle access to water transport, and also allowing the delivery of goods via the same.

Well: The well of a castle was vitally important in times of siege, as it could guarantee the survival of those inside. The well was sometimes located within the main keep so it could be protected, but could also be located near the kitchens for convenience.

Yett: A yett is much like a portcullis, in that it is latticed and used to protect an entrance. But a yett, unlike the portcullis, opens like a gate or door, rather than moving up and down vertically.

Again, this is only for reference, and if you're writing historical fiction you'll probably need more detailed information, including which parts of a castle were common for your time period and location. But this should give you something to go on. :)