Dai Viet ties its identity to Fortifications and tropical terrain, reflecting the historical Đại Việt kingdom's pattern of resisting invasion while expanding into Cham lands to the south.
"Hịch Tướng Sĩ": constructing a Fortification grants Culture scaled to that Settlement's Urban Population (15 Culture per point at its Exploration Age Apex tier, rising to 25 per point plus a Production bonus toward the Thành Huế Wonder once Dai Viet reaches the Modern Age as a Time-Tested civ), with a further Culture and Food bonus on Fortification Constructables built in Tropical terrain.
The Voi Chiến is a unique ranged unit (a heavy archer replacement) with increased Combat Strength, +1 Movement, +1 Sight, and the ability to move after attacking — though it costs more Production than the unit it replaces. Dai Viet also has a unique civilian unit, the Quân Vương, a Settler replacement that grants a burst of Culture for every tropical tile within 3 tiles of the city center it founds — making it worth planning your expansion route around before you build one.
The Water Puppet Theater is Dai Viet's unique infrastructure, protecting nearby Buildings from Flood damage and converting natural disasters into Culture instead of pure loss. Its associated Wonder, the Thành Huế, requires a City with a set number of Medieval Walls already built, acts as a Fortified District that enemies must conquer rather than raze, and grants an increased Specialist Limit to cities with enough Fortifications (though this resets on every Age Transition).