#royal setting

Castle

residence for nobility with fortifications and military structure

symbolical display of military power

often positioned strategically (on mountains or lakes, in river bends, …)

Fortress/Stronghold

purely functional defensive military outpost

usually in/near border regions for military to operate out of, lead offenses from and shelter retreating forces

Citadel

the most fortified area of a town or city (defensive core & last line of defense)

in form of a castle, fortress or simply a fortified center (defended through walls, elevation a maut, …)

houses military command, food storage and in time of war royalty

Keep

strongest, most defensible part of a castle in form of a tower

became highly uieffective with the invention of improved artilliary, as it became an easy target for cannons

alternate version would be a "bergfried," a tower with essentially the same function but not meant for permanent residence, only as a shelter during active invasion

Manor House

residence of a noble overseeing their lands and local peasants

mainly differs in function only

can be architectually very similar to a castle or could also be more similar to a farmhouse

typically no strong fortification, but often enclosed within walls or ditches

Bastion

a structure projective outward from the walls of a fortification, giving artillery better defensive positions and a greater range

contrary to a lot of modern (and even historic usage) a bastion is merely an architectual element, not its own kind of building

Oubliette

narrow pits accessible only through a hatch in the high ceiling

prisoners either left to starve or given just enough food to survive for months of even years, alone in the darkness

after the prisoner's death the body was either removed or kept, to be discovered by the next prisoner

used for those that the captors wish to forget

Prison Apartments

typically for royals or nobles who are kept under house arrest in apartments instead of a cell

usually just normal rooms with guards positioned outside; in some cases prisoners were even allowed to move around the castle but not leave it

Torture Chamber

attached to the cells and equipped for the purpose of torture

somestimes made up of several chambers, in which prisoners would be taken from one to the next with progressively worsening conditions

a lot of the torture devices we think off when thinking back to such chambers have no record of ever being actually used (eg. the iron maiden)