The outer defense of a castle or walled city; especially a double tower above a gate or drawbridge.
This feels to me like another Jack Vance word – that dude was always throwing around obscure architecture words and vehicle words and such. “Barbican” fits snugly into that list of things that I can immediately visualize but either didn’t know the word for or didn’t realize there was a specific word for. I’m pretty sure I had a barbican toy, with a little gate, when I was a kid.
Deriving from the Old French barbacane, for “exterior fortification,” the ultimate origin of the word isn’t known for sure, though it seems to be a good bet that it’s from the Arabic word for gatehouse.
Wherever it’s from, “barbican” is the essential fortification word you didn’t know you needed to know!
The three dismounted; and, leaving the camels to await their return, they leapt lightly across the fissure. Grotara and Thirlain Ludoch carrying the sarcophagus, and Yanur bearing the wineskin, they passed beneath the shattered barbican.
– Clark Ashton Smith, “The Weaver in the Vault” (1934)