Edinburgh Castle is a fortress.
An example of a fortress is a wall made out of stone that protects a city.
Origin of fortressMiddle English forteresse ; from Old French ; from Vulgar Latin an unverified form fortaricia ; from Classical Latin fortis, strong: see fort
Origin of fortressMiddle English forteress, from Old French, from Medieval Latin fortalitia, from Latin fortis, strong; see bhergh-2 in Indo-European roots.
(third-person singular simple present fortresses, present participle fortressing, simple past and past participle fortressed)
Old French forteresce, from Medieval Latin