They are all large structures from the Middle Ages.

A Fort is a fortified military structure to defend against enemy attacks. A fortress is located just outside the city, preferably on a hill, for a good view of the area. In a fort there are only soldiers.

A Fortress is another word for fortress. They both mean the same thing.

A Citadel is a large fortress. Citadel comes from the French of citadelle, which in turn comes from the Italian cittadella. That is a diminutive for citta = city.

A fortress and a citadel were first and foremost a place of refuge from outside attacks. In the second instance, it was to keep the citizens under control in case of riots.

A castle is also a fortified building, but also has a residential function for the rich. Castle comes from the Latin word castellum.

A Palace is where a reigning Prince or Bishop lives. Castles’ primary function is to defend and less as a luxury. A Palace is the other way around. Palaces are first and foremost homes for the kings and queens. In other words, more of an office building, from which to govern.

A castle is a fortified house, also to protect against attacks from outside. Burcht comes from the German word Burg, which means fortified settlement. The protection is usually a double stone wall. With a canal in between, possibly with a drawbridge. There are towers in the outer walls, which serve as a viewpoint and refuge. Castles are less luxurious than castles. It was not until the 15th^century, when the defense was less and less necessary and luxury became important for prestige, that they started to call castles castles. The more luxury there was, such as ballrooms, guest rooms, living quarters, courtyards, beautifully landscaped gardens, the more prestige, status and power one had.

A castle is another word for castle. They both mean the same thing.

A Bastion is a part of a fort. It is a protruding part of the outer wall/defensive wall. Made of earth or stone. You have a hollow bastion and a full bastion, see image below