Bow your head, say a prayer and maybe even light a candle in Lille Cathedral. This splendid religious building may not always be accessible, so check ahead for hours if you'd like to go inside.
You'll have no end of sightseeing choices when visiting Lille Centre Ville. Lille Cathedral is a good place to kick off here, but there are numerous other worthwhile sights to appreciate. Lille Grand Square, Rihour Square, and Citadel of Lille are not far from the principal church and will fill in your day nicely.
