Over the centuries this sanctuary has expanded to incorporate a large complex of religious buildings that overlay its original shrine. The double-arched entrance vestibule at street level stands next to a distinctive octagonal bell tower built by Carlo I of Naples in 1282. As you descend the staircase inside, look for the 17th-century pilgrims' graffiti. The grotto/shrine where St Michael is said to have left a footprint in stone is located at the bottom of the staircase.
Because of St Michael's footprint, it became customary for pilgrims to carve outlines of their feet and hands into the stone. Etched Byzantine bronze and silver doors, cast in Constantinople in 1076, open into the grotto itself. Inside, a 16th-century statue of the Archangel Michael covers the site of St Michael's footprint. Audio guides cost €3, and it's €5 to get into the museum (or €7 for both together).