Fortifications are military constructions and buildings designed for defence in warfare and military bases. Humans have constructed defensive works for many thousands of years, in a variety of increasingly complex designs. The term is derived from the Latin fortis ("strong") and facere ("to make").

Many military installations are known as forts, although they are not always fortified. Larger forts may class as fortresses; smaller ones formerly often bore the name of fortalices. The word fortification can also refer to the practice of improving an area's defence with defensive works. City walls are fortifications but not necessarily called fortresses.

The art of setting out a military camp or constructing a fortification traditionally classifies as castramentation, since the time of the Roman legions. The art/science of laying siege to a fortification and of destroying it has the popular name of siegecraft or 'siege warfare' and the formal name of poliorcetics. In some texts this latter term also applies to the art of building a fortification.

Ivan IV Vasilyevich (Russian:  Ива́н Четвёртый, Васи́льевич​ (help·info), Ivan Chetvyorty, Vasilyevich; 25 August 1530 – 28 March [O.S. 18 March] 1584), known in English as Ivan the Terrible (Russian:  Ива́н Гро́зный​ (help·info), Ivan Grozny; lit. Fearsome), was Grand Prince of Moscow from 1533 until his death. His long reign saw the conquest of the Khanates of Kazan, Astrakhan, and Siberia, transforming Russia into a multiethnic and multiconfessional state spanning almost one billion acres, approximately 4,046,856 km² (1,562,500 sq mi). Ivan managed countless changes in the progression from a medieval state to an empire and emerging regional power, and became the first ruler to be crowned as Tsar of all Russia.

Historic sources present disparate accounts of Ivan's complex personality: he was described as intelligent and devout, yet given to rages and prone to episodic outbreaks of mental illness. On one such outburst Tsar beat and unpremeditatedly killed his groomed and chosen heir Ivan Ivanovich. This led Tsardom to be passed to Tsar's younger son: the weak and intellectually disabledFeodor I of Russia. Ivan's legacy is complex: he was an able diplomat, a patron of arts and trade, founder of the Russia's first Print Yard, but he is also remembered for his paranoiac suspiciousness and cruel persecution of nobility.