Rivoalto (Rialto) was in the Middle Ages the common name for the area we now call Venice, which then meant the Duchy of the Venetians.
Translation of the parts related to Venice of the “De Administrando Imperio” (c.950) by Constantine VII Porphyrogenitus.
The Birthday of Venice of is traditionally on March 25th, 421 CE, but this is far more legend than historical truth.