Basilika St. Martin
First mentioned in AD 793, this soaring Catholic church was built on the site of an older Roman temple. It's been destroyed and rebuilt a number of times since then, which has resulted in an interesting mix of architectural styles; the 11th-century crypt and Gothic and baroque furnishings make it worth a visit. It's dedicated to the 4th-century Martin of Tours, and you'll find the former patron saint of France depicted in carvings and frescoes throughout the church.