Windsor exists because of its castle, and the castle is reason enough to come. Windsor Castle is the oldest and largest occupied castle in the world, a royal residence for nearly a thousand years, and it dominates the town from every angle. The State Apartments, St George's Chapel (where Henry VIII is buried alongside Charles I and Jane Seymour), and the changing of the guard are all worth your time.
The town itself is pleasant if unremarkable — a compact centre of Georgian and Victorian buildings sloping down from the castle to the Thames. The Long Walk stretches three miles south from the castle through Windsor Great Park, a vast royal parkland that is open to the public and perfect for a long walk or a run. Look out for the Crooked House on the High Street and the absurd shortness of Queen Charlotte Street beside it.
Across the bridge is Eton, home to the famous school. The high street is worth a stroll — all old bookshops and tailors. Further afield, Runnymede is where King John sealed the Magna Carta in 1215, and Legoland Windsor is the family attraction on the south-west edge of town. Windsor is an easy day trip from London, about 50 minutes by train from Paddington via Slough.