A barbican (from Old French: barbacane) is a fortified outpost or fortified gateway, such as at an outer defense perimeter of a city or castle, or any tower situated over a gate or bridge which was used for defensive purposes.
In the Middle Ages, barbicans were typically situated outside the main line of defenses, and were connected to the city walls with a walled road called the neck. In the 15th century, with the improvement in siege tactics and artillery, barbicans lost their significance. However, several barbicans were built well into the 16th century.
Fortified or mock-fortified gatehouses remained a feature of ambitious French and English residences well into the 17th century.
Portuguese medieval fortification nomenclature uses barbican to describe any wall outside of and lower than the main defensive wall that forms a second barrier. The barrier may be complete, extensive or only protect particularly weak areas. They use the more restrictive term gate barbican for structures protecting gates.
Paul Deschamps (1888-1974) interpreted the Arabic word 'bashura[h]' as used in 13th-century chronicles to mean barbican, a defensive structure placed ahead of a gate, but this has been debunked, 'bashura' denoting rather an entire section of the outer fortifications, which may include a barbican, but also a bastion, gate, tower, or all of those together.
Fortifications in East Asia also feature similar high structures. In particular, gates in Chinese city walls were often defended by an additional "archery tower" in front of the main gatehouse, with the two towers connected by walls extending out from the main fortification. Literally called "jar walls", they are often referred to as "barbicans" in English.