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Frontier Fortresses is an Economic Policy Card from the Gifts of the Nile scenario in Civilization VI.
Strategy[]
Frontier Fortresses provides +4 
Gold from every 
Trade Route, much like Triangular Trade from the base game.
Civilopedia entry[]
"As our wealth grows, so do the enemies who wish to take it from us. A series of fortresses built at the edges of our civilization will warn us of approaching armies."