Fortified tower in the context of "Curtain wall (fortification)"

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.

Castillo San Felipe del Morro (English: Promontory Castle of Saint Philip), most commonly known as El Morro (The Promontory), is a large fortress and citadel in the Old San Juan historic quarter of San Juan, the capital municipality of Puerto Rico. Commissioned by King Charles I of Spain in 1539, it was first built as a fortified tower in honor of King Philip II, who oversaw its expansion into a hornwork bastion fort by 1595. Over the next 200 years, especially in the reign of King Charles III, El Morro continued to be developed to reach its current form in 1787. Rising 43 metres (140 ft) from the Atlantic shoreline with 5.5 to 7.6 metres (18 to 25 ft) thick walls, the six-leveled edifice stands on a steep, rocky headland promontory on San Juan Islet guarding the entry to San Juan Bay, the harbor of Old San Juan. El Morro, alongside La Fortaleza, San Cristóbal, El Cañuelo, and other forts part of the Walls of Old San Juan, protected strategically and militarily important Puerto Rico, or La Llave de las Indias (The Key to the Indies), from invasion by competing world powers during the Age of Discovery and Sail. It was designated a World Heritage Site by UNESCO in 1983.

Himeji Castle (姫路城, Himeji-jō; [çimeʑiꜜʑoː] ) is a hilltop Japanese castle complex situated in Himeji, a city in Hyōgo Prefecture, Japan. The castle is regarded as the finest surviving example of prototypical Japanese castle architecture, comprising a network of 83 rooms with advanced defensive systems from the feudal period. The castle is frequently known as Hakuro-jō or Shirasagi-jō ("White Egret Castle" or "White Heron Castle") because of its brilliant white exterior and supposed resemblance to a bird taking flight.

As with almost all Japanese castles from the Azuchi-Momoyama period onward, the tenshu (天守, main keep), the most prominent structure, was used as a storehouse in times of peace and as a fortified tower in times of war, and the daimyo (大名, feudal lord)'s government offices and residences were located in a group of single-story buildings near the tenshu and the surrounding yagura (櫓, turrets).

A mural crown (Latin: corona muralis) is a crown or headpiece representing city walls, towers, or fortresses. In classical antiquity, it was an emblem of tutelary deities who watched over a city, and among the Romans a military decoration. Later the mural crown developed into a symbol of European heraldry, mostly for cities and towns, and in the 19th and 20th centuries was used in some republican heraldry.