Fortification
Older fortification may prove useful such as the old 'Yugo' pyramid shelters built in the 1980s which were used by US personnel on 8 Jan 2020 when Iran fired 11 ballistic missiles at Ayn al-Asad Airbase in Iraq.
Fuel is volatile and has to comply with rules for storage which provide protection against accidents. Fuel in underground bulk fuel installations is well protected though valves and controls are vulnerable to enemy action. Above-ground tanks can be susceptible to attack.
Ground support equipment will need to be protected by fortifications to be usable after an enemy attack.
Permanent (concrete) guard fortifications are safer, stronger, last longer and are more cost-effective than sandbag fortifications. Prefabricated positions can be made from concrete culvert sections. The British Yarnold Bunker is made from sections of a concrete pipe.
Guard towers provide an increased field of view but a lower level of protection.
Dispersal and camouflage of assets can supplement fortifications against some forms of airfield attack.

Counter-insurgency: Just as in colonial periods, comparatively obsolete fortifications are still used for low-intensity conflicts. Such fortifications range in size from small patrol bases or forward operating bases up to huge airbases such as Camp Bastion/Leatherneck in Afghanistan. Much like in the 18th and 19th century, because the enemy is not a powerful military force with the heavy weaponry required to destroy fortifications, walls of gabion, sandbag or even simple mud can provide protection against small arms and anti-tank weapons—although such fortifications are still vulnerable to mortar and artillery fire.

Forts: Forts in modern American usage often refer to space set aside by governments for a permanent military facility; these often do not have any actual fortifications, and can have specializations (military barracks, administration, medical facilities, or intelligence).
However, there are some modern fortifications that are referred to as forts. These are typically small semi-permanent fortifications. In urban combat, they are built by upgrading existing structures such as houses or public buildings. In field warfare they are often log, sandbag or gabion type construction.
Such forts are typically only used in low-level conflicts, such as counterinsurgency conflicts or very low-level conventional conflicts, such as the Indonesia–Malaysia confrontation, which saw the use of log forts for use by forward platoons and companies. The reason for this is that static above-ground forts cannot survive modern direct or indirect fire weapons larger than mortars, RPGs and small arms.

Prisons and others: Fortifications designed to keep the inhabitants of a facility in rather than attacker out can also be found, in prisons, concentration camps, and other such facilities. Those are covered in other articles, as most prisons and concentration camps are not primarily military forts (although forts, camps, and garrison towns have been used as prisons and/or concentration camps; such as Theresienstadt, Guantanamo Bay detention camp and the Tower of London for example).

Field fortifications: 

Notes: 

References: This article incorporates text from a publication now in the public domain: Jackson, Louis Charles (1911). "Fortification and Siegecraft". In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 10 (11th ed.). Cambridge University Press. pp. 679–725.

Bibliography: July, Robert Pre-Colonial Africa, Charles Scribner, 1975.
Murray, Nicholas. "The Development of Fortifications", The Encyclopedia of War, Gordon Martel (ed.). WileyBlackwell, 2011.
Murray, Nicholas. The Rocky Road to the Great War: The Evolution of Trench Warfare to 1914. Potomac Books Inc. (an imprint of the University of Nebraska Press), 2013.
Osadolor, Osarhieme Benson, "The Military System of Benin Kingdom 1440–1897", (UD), Hamburg University: 2001 copy.
Thornton, John Kelly Warfare in Atlantic Africa, 1500–1800, Routledge: 1999, ISBN 1857283937.

External links: 
Fortress Study Group
Military Architecture at the Wayback Machine (archived 5 December 2018)
ICOFORT