Barbican

n. fortified tower, fortified bridge

A barbican (from mediæval Latin barbecana) is a fortified outpost or gateway, such as an outer defence to a city or castle, or any tower situated over a gate or bridge which was used for defensive purposes. Usually barbicans were situated outside the main line of defences and connected to the city walls with a walled road called the neck.
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Noun
1. a tower that is part of a defensive structure (such as a castle)
(synonym) barbacan
(hypernym) tower

(n.)
Alt. of Barbacan
  

An approach to a town or castle which is defended by a field of fire from one or more towers, generally two. The barbican itself, an outcropping from the gatehouse , allowed approach to the gate only through a narrow, easily defended passage.