The Square Mile (gold outline). The original Roman settlement — one square mile, governed by its own Corporation for 2,000 years. Financial capital of England since the Middle Ages.

Invasion & Defence. Tower Hill to All Hallows: Roman wall, Norman castle, and the oldest church in the City. Three layers of conquest in 200 metres.

Trade & Commerce. The Monument, Cannon Street, St Paul's — the City that burned in 1666 and rebuilt itself in a generation. Lloyd's of London born from a coffee-house conversation nearby.

Finish at Temple Bar. St Bride's on Fleet Street is the City of London's last stop before the boundary into Westminster — your Tour 2 starting point.

Shortest sea crossing. Boulogne across to Dover — just 33 miles — was the shortest route from a Roman port across the Channel, making it the obvious crossing for Roman supply ships.

Tidal river as a highway. The Thames bore ships inland on the flood tide, then carried them back out on the ebb — no rowing required for most of the journey.

Natural crossing point. Londinium sat on the first gravel bank above the tidal marshes — wide enough to bridge, shallow enough to ford, and far enough from the sea to be defensible.

Protected by marshes. Surrounding wetlands made the city hard to attack overland, while the river kept it open to resupply and trade.