Florence is the Assassin's Creed II city associated with the Ponte Vecchio and Palazzo Vecchio.
Assassin's Creed II begins in Renaissance Italy, where Ezio Auditore da Firenze grows from a young Florentine nobleman into an Assassin. Florence is one of the game's principal locations, and its streets, rooftops, churches, and landmarks provide the setting for major story missions and free exploration.
The Ponte Vecchio is a historic bridge crossing the Arno River, while the Palazzo Vecchio is the fortress-like town hall in Piazza della Signoria. The game adapts recognizable landmarks rather than presenting a museum-accurate reconstruction of every building. Players can also visit other Italian locations, including Venice, Monteriggioni, Forlì, and the Papal capital of Rome during the broader story.
A common mix-up is choosing Venice because it is another highly memorable Assassin's Creed II city. Venice is especially prominent later in the game, but Florence is the correct city for both landmarks named in the question.