Horse Guards is the Household Cavalry's building in Westminster, a database landmark in Syndicate.
The site was once the palace tiltyard, used for jousts and bear-baiting. Its basement later held a cockfighting pit and its coffee house drew prostitutes until 1850. In 1861 the Assassin Simeon Price stood trial for desertion here and was cleared.
The first building of 1665 decayed until people inside risked being hit by falling masonry. Police called to the later cockfighting pit and coffee house were often sent away by the soldiers.
10 Downing Street
10 Downing Street is the Prime Minister's home and office in Westminster. In 1868 Benjamin Disraeli lives here, and Jacob's work as his bodyguard in Sequence 7 starts outside the house.
Admiralty
The Admiralty is a government office on Whitehall in Westminster, one of the period buildings you can climb and read about in the in-game database. No memory centres on it.
Banqueting Hall
The Banqueting Hall, later the Chapel Royal, is a 1622 hall on Whitehall in Westminster and a database landmark in Syndicate. It is the only surviving part of the old Palace of Whitehall.
Blue Anchor Alley
Blue Anchor Alley is a Blighter stronghold in Westminster run under Lilla Graves, one of two in that borough. Freeing it lets the Rooks set up there instead.