Which Assassin's Creed game introduced the series' first fully explorable version of London?

Assassin's Creed Syndicate introduced the series' first fully explorable version of London.

Released in 2015, Syndicate is set in London during the Industrial Revolution. Players control the twin protagonists Jacob and Evie Frye as they fight Templar influence and build an organized gang in the capital. The open world includes districts such as Westminster, the City of London, and Whitechapel.

Syndicate added a grappling hook, allowing the Frye twins to cross streets and reach rooftops more quickly. It also introduced drivable carriages and missions involving the expansion of the Rooks, Jacob's allied gang.

Assassin's Creed Unity is set in Paris, not London. London later appeared in other franchise material, but Syndicate is the main Assassin's Creed title built around a broad, explorable Victorian-era London. The city is therefore a setting clue that distinguishes Syndicate from Unity and the earlier Italian games.