Castellum in the context of Hadrian's Wall

A fortification (also called a fort, fortress, fastness, or stronghold) is a military construction designed for the defense of territories in warfare, and is used to establish rule in a region during peacetime. The term is derived from Latin fortis ("strong") and facere ("to make").

From very early history to modern times, defensive walls have often been necessary for cities to survive in an ever-changing world of invasion and conquest. Some settlements in the Indus Valley Civilization were the first small cities to be fortified. In ancient Greece, large cyclopean stone walls fitted without mortar had been built in Mycenaean Greece, such as the ancient site of Mycenae. A Greek phrourion was a fortified collection of buildings used as a military garrison, and is the equivalent of the Roman castellum or fortress. These constructions mainly served the purpose of a watch tower, to guard certain roads, passes, and borders. Though smaller than a real fortress, they acted as a border guard rather than a real strongpoint to watch and maintain the border.

The Château de Brest (Breton: Kastell Brest, pronounced [ˈkastɛl bʀest]) or Castle of Brest is a castle in Brest, Finistère, Brittany, France. The oldest monument in the town, it is located at the mouth of the river Penfeld at the heart of the roadstead of Brest, one of the largest roadsteads in the world. From the Roman castellum to Vauban's citadel, the site has over 1700 years of history, holding right up to the present day its original role as a military fortress and a strategic location of the highest importance. It is thus the oldest castle in the world still in use, and was classified as a monument historique on 21 March 1923.

The structure's heterogeneous architecture has been the result of continual adaptations to developments in siege warfare and armament on land and sea. The château stands on the opposite bank to the Tour Tanguy combining to defend the entrance to the Penfeld.

The Kromme Rijn (Dutch pronunciation: [ˈkrɔmə ˈrɛin]; "Crooked Rhine", for its many bends) is a river in the central Netherlands.

In Roman times, this northernmost branch of the Rhine delta was the main distributary of this major European river. Along its banks the Romans built their frontier castella part of the Limes Germanicus.