Henry V set off to France from Portchester Castle in 1415. The expedition ended in the victorious Battle of Agincourt and Henry's marriage into the French royal family.

Portchester's greatest period of service was undoubtedly during the Hundred Years' War with France, which began in 1337. Henry's military expedition was the last of many which crossed to France from here. The Romans built a naval base here in the third century to guard against attack by the Barbarians. Today the walls are the most complete in Europe, with spectacular views over the castle from the Norman keep.