The headquarters of the City of London Corporation, the administrative body that has overseen the City's affairs for the past 800 years, the Guildhall's original medieval framework has endured significant repairs following the 1666 Great Fire and World War II (as well as the addition of a rather incongruous concrete wing in the 1970s). Today its Great Hall has a touch of the medieval theme park about it, filled with colorful livery banners and with a (reconstructed) minstrel's gallery from where 3m (9-ft.) statues of mythical giants Gog and Magog gaze down on proceedings.
East across Guildhall Yard, the Guildhall Art Gallery displays a constantly updated selection from the Corporation's 4,000-plus works relating to the capital. The main attraction (there's certainly no missing it) is John Singleton Copley's The Defeat of the Floating Batteries at Gibraltar, September 1782, which at 42.5 sq m (458 sq ft.) is Britain's largest independent oil painting, and takes up two whole stories. Fridays are the best time to visit, when admission is free, and you can join a free tour of the collection at 12:15, 1:15, 2:15, and 3:15pm.
Just down the corridor from the library is the one-room Clockmaker's Museum (www.clockmakers.org), filled with tick-tocking timepieces from throughout history, including John Harrison's 18th-century H5 marine chronometer (the "longitude" clock), the world's first electric clock, and the watch worn by Edmund Hillary on his 1953 ascent of Everest. It's also free and is open Monday to Saturday from 9:30am to 5pm.
Head down beneath the gallery to visit the scant remains of London's Roman amphitheater, which dates from the 2nd century A.D. but remained undiscovered until 1988 (and didn't go on display until 2003). Images of spectators and missing bits have been added to give visitors a better idea of what it once looked like.