Curtain wall (fortification) in the context of "North Wales"

A castle is a type of fortified structure built during the Middle Ages predominantly by the nobility or royalty and by military orders. Scholars usually consider a castle to be the private fortified residence of a lord or noble. This is distinct from a mansion, palace, and villa, whose main purpose was exclusively for pleasance and are not primarily fortresses but may be fortified. Use of the term has varied over time and, sometimes, has also been applied to structures such as hill forts and 19th- and 20th-century homes built to resemble castles. Over the Middle Ages, when genuine castles were built, they took on a great many forms with many different features, although some, such as curtain walls, arrowslits, and portcullises, were commonplace.

European-style castles originated in the 9th and 10th centuries after the fall of the Carolingian Empire, which resulted in its territory being divided among individual lords and princes. These nobles built castles to control the area immediately surrounding them and they were both offensive and defensive structures: they provided a base from which raids could be launched as well as offering protection from enemies. Although their military origins are often emphasised in castle studies, the structures also served as centres of administration and symbols of power. Urban castles were used to control the local populace and important travel routes, and rural castles were often situated near features that were integral to life in the community, such as mills, fertile land, or a water source.

A bastion fort or trace italienne (a phrase derived from non-standard French, meaning 'Italian outline') is a fortification in a style developed during the early modern period in response to the ascendancy of gunpowder weapons such as cannon, which rendered earlier medieval approaches to fortification obsolete. It appeared in the mid-fifteenth century in Italy. Some types, especially when combined with ravelins and other outworks, resembled the related star fort of the same era.

The design of the fort is normally a polygon with bastions at the corners of the walls. These outcroppings eliminated protected blind spots, called "dead zones", and allowed fire along the curtain wall from positions protected from direct fire. Many bastion forts also feature cavaliers, which are raised secondary structures based entirely inside the primary structure.

A bastion is a structure projecting outward from the curtain wall of a fortification, most commonly angular in shape and positioned at the corners of the fort. The fully developed bastion consists of two faces and two flanks, with fire from the flanks being able to protect the curtain wall and the adjacent bastions. Compared with the medieval fortified towers they replaced, bastion fortifications offered a greater degree of passive resistance and more scope for ranged defence in the age of gunpowder artillery. As military architecture, the bastion is one element in the style of fortification dominant from the mid 16th to mid 19th centuries.

A defensive wall is a fortification usually used to protect a city, town or other settlement from potential aggressors. The walls can range from simple palisades or earthworks to extensive military fortifications such as curtain walls with towers, bastions and gates for access to the city. From ancient to modern times, they were used to enclose settlements. Generally, these are referred to as city walls or town walls, although there were also walls, such as the Great Wall of China, Walls of Benin, Hadrian's Wall, Anastasian Wall, and the Atlantic Wall, which extended far beyond the borders of a city and were used to enclose regions or mark territorial boundaries. In mountainous terrain, defensive walls such as letzis were used in combination with castles to seal valleys from potential attack. Beyond their defensive utility, many walls also had important symbolic functions – representing the status and independence of the communities they embraced.

Existing ancient walls are almost always masonry structures, although brick and timber-built variants are also known. Depending on the topography of the area surrounding the city or the settlement the wall is intended to protect, elements of the terrain such as rivers or coastlines may be incorporated in order to make the wall more effective.

Lancaster Castle is a medieval castle and former prison in Lancaster in the English county of Lancashire. Its early history is unclear, but it may have been founded in the 11th century on the site of a Roman fort overlooking a crossing of the River Lune. In 1164 the Honour of Lancaster, including the castle, came under royal control. In 1322 and 1389 the Scots invaded England, progressing as far as Lancaster and damaging the castle. It was not to see military action again until the English Civil War. The castle was first used as a prison in 1196 although this aspect became more important during the English Civil War. The castle buildings are owned by the British sovereign as Duke of Lancaster; part of the structure is used to host sittings of the Crown Court.

Until 2011 the majority of the buildings were leased to the Ministry of Justice as HM Prison Lancaster, after which the castle was returned to the Duchy's management. The castle is now open to the public seven days a week and is undergoing a large-scale refurbishment. There is a large sweeping public piazza, allowing access to the cloistered area, renovated in 2019. A new section of the café has been built, against the old outer curtain wall, which was reduced in height to afford views of the neighbouring Lancaster Priory. This is the first 21st-century addition to the castle. Another renovated building adjoining the café is leased to Lancaster University as a campus in the city with small conference facilities.