Silla is built entirely around Alliances — the strongest diplomatic-economic hybrid of the Antiquity Age civs, but one that genuinely struggles if your starting continent leaves you with nobody to ally.
"Maripgan" grants +1 Culture on Resources as its baseline. Unlocking the right Civics adds much more: forming an Alliance gives both leaders a free Trade Route from the other's Capital that persists for the Alliance's duration, plus increasing Influence for every civilization you have at least one Trade Route with (rising from +1 to +2 to +3 per civilization across successive tiers).
The Hwarang is a unique Ranged unit with increased Movement and Attack Range that counts as a Cavalry unit for upgrade purposes. The Sangdaedeung is a unique Merchant unit that grants Gold per positive Relationship Level with a leader when it creates a Trade Route — making peaceful diplomacy directly profitable rather than just safe.
Silla's unique buildings are the Lecture Hall (Culture, with a Resource Capacity bonus on Rough Terrain) and the Pagoda (Happiness, with Influence per adjacent Mountain, Natural Wonder, or Wonder), combining into the Sachal Unique Quarter which adds Gold per Resource slotted in the city. The associated Wonder, Emile Bell, adds Influence and unlocks a unique Diplomatic Endeavor that grants Food to both leaders' Capitals.